The Silmarillion was a collection of world-generation notes by JRR Tolkien. Just like a modern passionate DM would do, he wrote many short stories about his own personal world inside of a journal. Most of it was incoherent and disorganized, as he wrote to it whenever whim took him in a writing mood. Of course, Tolkien died before he could ever put any of this to a book, so his journal was stuffed into a box and forgotten.
... but, like the Ring of Sauron, the journal was not lost. Many years later, his son found it and decided to create a compilation of all the short stories. While there were many errors with timelines and such (Tolkien did not actually write it in any semblance of chronological order), his son made a great effort to organize it into a coherent work. Thus: the Silmarillion. It is widely considered a...difficult read to say the least and is very often considered very boring (because, as mentioned before, he died before he could assemble it into a coherent narrative). It's definitely written more like a collection of mythological tales like you might find for, say, Greek myths in a school library (only less accessible) than it is like a novel.
Or, more to the point, like the Bible. Tolkien was a Catholic. Many lines could be drawn between his own creation myth with that of Christianity's. You could easily say that Eru Ilúvatar was equivalent to God, and Melkor would be Satan (He's even described as being the most beautiful of the Valar!).
In A Nutshell
The Creation Myth
In the beginning, there was absolutely nothing. Then the god Eru Ilúvatar began singing. Of course this wasn't just any sort of singing, because to gods like him, singing was a means of weaving raw magic into a pure and physical form. He began his singing by creating his children, the Ainur, who in turn joined him in a new heavenly chorus. There were many Ainur, who were divided into the older and more powerful Valar, and the younger and less powerful Maiar. (Many better-known LOTR characters that are Maiar include Gandalf, Saruman, the Balrogs, and Sauron.) The Ainur each sang a different part of the world into existence, such as Ulmo who created the seas and oceans, the only thing Ilúvatar added was the Secret Fire, the source of life that only he possessed. The Ainur continued singing various into existence, including Aulë who created the Dwarves.
Long dramatic pause.
Ilúvatar found out about the dorfs and was displeased, as only he had the ability to genuinely sapient creatures. He had a vision of the world he was trying to create and his vision didn't include stumpy, uncouth beard people running around with picks and axes grunting "Genocide! Mining!". Aulë was ashamed, explaining he'd made them due to love for Ilúvatar, but resolved to destroy them. Ilúvatar saw Aulë's creations plead for mercy and stayed Aulë's hammer, allowing the dorfs to stay and refining Aulë's work by infusing the dorfs with the Secret Fire to give them true life. Then Aulë's wife Yavanna found out the dorfs would have axes and that the elves would be no good in keeping them from her trees so she made some angry sentient trees of her own to fight her hubby's dorfs. How Ilúvatar handled hearing about THAT is not explored, but can't have been too bad since the Ents were allowed to stay. The dorfs would later wake up, around the mid years of Melkor's groundment.
But their antics were nothing compared to the troublemaker of the group. An Ainur by the name of Melkor; described as the most beautiful and gifted singer of all the Ainur, decided that he wanted to create his own song. At first he was simply an arrogant prick, as he hated collaboration and wanted something that was wholly his own creation. Twice in a row he tried this, and each time Ilúvatar showed him a new, more beautiful song to both amaze the other Ainur, and show up Melkor (because attempting to humble selfish people by upstaging them never, ever backfires). Melkor first created Fire, hot and cold. As a consequence of the creation of Fire, evil was introduced into the world, but also beauty was created as hot and cold made Weather. The second of his singing served to seat evil in the world by warping many creations, such as granting scorpions their tails. For this, Melkor was cast out of the Ainur's Chorus. Embittered and emboldened by his failure, he continued to desire the Secret Fire for himself; lacking the ability to create life on his own, he instead chose to infuse his own power into Arda, attempting to corrupt all of creation toward his will. At this, he was largely successful, even though the very act drained him of power until he feared even the power of mortals. As a result of this his influence lingers on in the world even long after his defeat; his dark power now so dispersed that nothing in Arda is truly free of its corruption. It's particularly concentrated in Gold, which is why dragons love the stuff so much. Then Ilúvatar stepped back and appointed the Valar - sans Melkor - to run Middle-Earth, only personally getting involved when the Valar asked for help, things went really pear-shaped or something really beautiful happened.
The Valar that still wanted to control everything got sick of Melkor's shit after he destroyed the two lights they were using to keep Arda (the Earth) from being nothing but shades of grey with chest-high walls. These lamps blew up when Melkor knocked them over, and split the formerly continuous landmass into three separate continents. So they decided that if Melkor was going to be a little bitch and destroy shit, they might as well turtle up in their own corner of the world. And so they went to the western continent (called Aman), and resolved to Make Aman Great Again by building a giant fuckoff wall of mountains to keep out all the
Mexicans middle earthers, and growing a couple glowing trees for mood lighting. Eventually they got around to making stars, and managed to locate the Elves. Although not before Melkor found them first and took a few. Then Tulkas came out with a folding chair, threw Melkor out of the ring and dragged him back as a captive (but not before he had a chance to corrupt some Elves, essentially turning them into Orcs. Maybe.). Elves had spawned in Middle Earth (the central continent), but the Valar invited them to Aman in what was later to be dubbed "worst idea ever".
Most of the Elves took the offer and for a while things were working out really well as they applied their creativity to all the crafting skills the Valar taught them. Fëanor, an elf prince, made three awesome jewels called the Silmarils that glowed just like the two trees used to light up the place. However, fucking Melkor decided to be a sneaky prick. He pretended to apologize to the Valar, and asked if he could see what they were working on. Caught off-guard by his apology, they accepted. He immediately smote the two trees, killing them, stole the Silmarils and fucked off to Middle-Earth to raid the elves there. He got his ass handed to him again, and holed up in the roguelike dungeon expansion pack Angband, due to his old Utumno version being too resource-intensive. Then Melkor went on a crafting spree, creating beings such as werewolves and vampires for any Maia who helped him and started making dragons for himself who, who for some reason, took longer to make. He then spammed the Maia with messages and gave some of the ones who joined him the gift of being "Balrogs", while the first dragon - code-named "Glaurung" - was successful enough to start testing on his enemies. But Glaurung attacked Ard-galen too early, due to Melkor being a fail, which alerted the Valar to Melkor's plans. Annoyed, Melkor managed to retrieve and perfect Glaurung, and used him as the prototype to create more dragons, the largest by far being Ancalagon. Once finished, he spawned an army of them to rampage across Middle-Earth before multiple factions joined forces and thwarted Melkor's army, in an event later known as the Battle of Unnumbered Tears, though Ancalagon wasn't slain until the War of Wrath.
Fëanor was fucking pissed, so much that it would make Angry Marines look like pussies, and kept calling Melkor "Morgoth." He was so pissed he picked a fight with another elf kingdom when they said "Dude, just chill". His sons would later pick up his teamkilling habits, the greedy little psychos. He travelled to Middle-Earth, raiding Angband over and over again to try and get his light-jewels back, and got himself killed. His elf kingdom stayed there, split between his sons, his half-brother, his half-brother's sons, his other half-brother's sons, and a king named Thingol who wasn't related to him, and who was understandably salty as fuck when he found out about the teamkilling of his related kin. They kept attacking Angband for 400 years until Morgoth said "enough already!" and got those damn kids off his front lawn.
Meanwhile, back in Aman, the Valar hastily tried to replace the lost light tree thingies with a star. This new 'sun' only works half the time, but it's better than nothing. (If you say "they could've used the Silmaril", those got stolen so what're you gonna do?) Besides, the bloody elves really liked the stars and would've bitched for eons if they got completely replaced. It was also around this time that Ilúvatar's second batch of creations woke up, a watered down version of Elves called Men. At some point Men got their first experience with encountering Angband and Morgoth. Some worshipped Morgoth (the Original Sin for humanity in Middle-Earth), but others saw how much of That Guy he was and got away ASAP.
Then the DM made a character for himself, named Beren, and started a romance with an elf king's daughter Lúthien (the Dungeon-master's-girlfriend's character). Elf-dad Thingol was a cultural elitist and a snob, but that would look bad, so he said "sure you can date my daughter, IF you can finish a run of Angband by bringing a Silmaril jewel back to me." Beren and Lúthien did the quest together, almost got wrecked by a Maiar named Sauron (more on him later), but managed to pull it off. On the way back they encountered Carcharoth, the world's mightiest werewolf, so Lúthien hit him with a sleep spell... but he woke up before the spell ended so Beren tried to fend him off with the Simaril's light. Instead, Carcharoth bit off Beren's hand and swallowed it along with the Simaril, but touching Simarils is agonizing for anything evil (they scarred Melkor's hands) so Carcharoth was wracked with so much pain that he fled. The wedding was on a Tuesday, and afterwards Beren led a raid to kill Carcharoth and get the god damned Simaril back, which they accomplished. But Beren and a heroic Valar-blessed dog got killed by Carcharoth's poison bite. Lúthien later became an heroine... but she sang some Goth poetry to the Grim Reaper-equivalent who, with Ilúvatar's permission, gives them an extra life for finding the Simiaril, and they both live as mortals until they died a second time.
The rest of the grey elves were pretty sure that a Man and an Elf playing together was some kind of exploit, so they actually started treating them well, just like they had kindly treated the Dwarves, at that time. Unfortunately, Melkor was able to appeal to the inherent evil and greed of Men and got them to do all kinds of stupid shit. Still, not every Man was an idiot, some had great skill or wisdom, so the elves didn't go back to being total pricks. Some men picked up Morgoth's trollish habits and became his underlings, but there were also three bro-tier clans of men closely allied to the elves and fought on the front lines of the siege, who were collectively known as the Edain.
Blah blah a Man named Túrin leaves home, blah blah blah loses his memory and hooks up with his mind-wiped sister, blah blah defeats the dragon Summer Glaurung, totally not his fault he killed his best friend, totally not his fault he knocks up his own sister, when they both find out they become an heroic pair. This was expanded on in the book The Children of Húrin, published well after Tolkien himself died. It started with Túrin's dad Húrin, who was captured by Morgoth after a battle but had the massive steel balls to tell Morgoth to piss off to his face when the latter tried to get Húrin to sell out a hidden elf city so Morgoth could purge them. Morgoth, like a GM angry no one wanted to play by his rules, cursed his whole family resulting in a story bleaker, but better than Game of Thrones. That's not a lie either, Túrin goes through some hard shit in his life before he dies, including his mother losing her home and her mind and Húrin is restrained "And I Must Scream" style and forced to see only the worst moments of his children lives. You thought the Starks had it tough? Think again! Morgoth, having had his Butthurt over Húrin insulting him, releases him several decades after the big battle where there were a lot of tears and Húrin was captured. Húrin, pissed off beyond belief, goes to Gondolin and asks for Turgon's help, who doesn't even let him in the barrier. So he turns towards it and tells Turgon to go fuck himself like the ungrateful bastard he is, which causes a change of heart on Turgon and so dispatches eagles to pick him up but he leaves. Morgoth, watching Húrin, finally learns the general direction of Turgon's Hidden City. Húrin goes to the grave of Túrin, where his wife Morwen is mourning all three of their children; Húrin barely catches her last moments to have one last talk and buries her as well. Húrin goes to Nargothrond and kills the last Petty-dwarf Mîm (a sub-race of Dwarfs) for betraying Túrin (though Túrin's gang had killed one of his sons by accident; his other son and most of Túrin's gang were wiped out by an Orc war party), takes the super Dwarf necklace Nauglamír from the broken elven city, goes to Doriath and gives it Thingol, insulting him by saying that it's the price for keeping his children safe. Melian shows him pity and uses her angel powers to unfuck his mind, at which point he apologizes and sees how his rage-fuelled mini quest only helped Morgoth. He formally thanks Thingol and leaves to kill himself by jumping into the sea.
Bro-tier Man and war hero named Tuor (Túrin's cousin, Húrin's nephew and Huor's son, you can easily see which side of the family got all the luck) weds a hot elf shortie named Idril, after a Vala tells him how to get to the Elven version of Seattle, which is where the elves move to after the Angband MMO servers shut down and they lost all their kingdoms. They have a son they name Eärendil. Naturally, since this is a tragedy, Elf Seattle also falls when Eärendil's a kid, all because Idril's creepy cousin wanted to bone her. Again, incest is most definitely not wincest. Together with many other fleeing elves Eärendil ends up south in what was essentially a giant Elf refugee camp, since at this point Morgoth had ruined nearly every other Elf realm further north. Tuor and Idril's kid Eärendil ends up dating Beren & Lúthien's grandkid Elwing, who has a Silmaril from her grandparents' adventures. Eärendil and Elwing end up with two kids, Elrond and Elros, but due to the way Eru wrote the code for souls and metaphysics the kids had to choose to be either elven or human. Later, some of Fëanor's sons, still wanting to reclaim their family bling, attack the Elf refugee camp, because they know Elwing has it. Eärendil was out at sea while this happened, but Elwing threw herself into the ocean with the Silmaril and transformed into a bird to escape, leaving her kids behind. It worked out though, because one of Fëanor's sons suddenly grew a conscience and decided to take care of them.
Eärendil, sick of how everything was getting steadily more grimdark, sails to the western continent with his wife to petition the Valar for aid. Moved, they break forth their wrath and smite down Morgoth. They smite him so hard that the continent they were playing on, Beleriand, broke and sank into the ocean. The remaining two Silmarils were also brought out, and Feanor's two remaining sons saw their chance for an easy attack on Angband. They're told it's a bad idea, they have a quick discussion about whether they'll get banned for this, then raid the camp and pull off the quest, but when they try to use the Silmarils they find they've become evil, so trying to hold the gems hurts them. The older brother Maedhros kills himself, while Maglor goes off to an unknown fate, finally bringing Fëanor's kind to an end. Eärendil is allowed a choice on whether to be an Elf or human, he chooses elfhood for the sake of his wife who chose to be an Elf. Their sons also choose, Elrond stays and Elf and Elros chooses humanity. The Silmarils thus find their places, Maedhros having tossed himself into the ground while holding one, Maglor tossing his into the ocean before disappearing and Eärendil taking his across the heavens on his nightly voyages. Eventually one of the Silmarils maybe gets dug up by the Dorfs in the Lonely Mountain... the continuity is kinda unclear cuz this book was mostly notes and half-formed ideas.
Morgoth is currently removed out of, and forbidden from entering, Middle Earth, though there's rumours that he'll eventually come back. There's also talk that this will happen when Ilúvatar decide to rebuild the world and fix Morgoth's damage to the world, and then Morgoth will be defeated once and for all.
The last stories concern the rings of power and the fall of Númenor and (most of) its Dúnedain. Technically we're on to the Second Age so no longer in Silmaril scope.
On the way back home from wrecking Melkor's shit, the Ainur were impressed with three kingdoms of Western Man ("Dúnedain") who'd joined the elvish bro-quest (skipping the late-coming fourth loyalists, the sons of Bór, because Tolkien forgot Matthew 20:1–16). The Valar gave these Men and Women an island Númenor to mutually sex each other in, with plenty of tech and weapons they could need when they ever crawled out of bed. Elros is named first king for his descent from human and elven royalty as well as having a bit of Angel blood through Melian. Over long generations the now-mixed Númenóreans got all human nationalist, at least the Kings and the Kings' Men; as Pratchett observes, "black and white live in harmony to gang up against green". Other Númenóreans stayed Faithful to the ways of elves and Valar.
Meanwhile, not all Melkor's lieutenant Maiar had been snapped up in the Valars' dragnet after the Big Guy got his sorry ass imprisoned. The Balrogs, being ugly, had no choice but to flee into the deep places of the earth. But Sauron had maintained his own prettiness. He fooled men and, apparently, angels into thinking he was going to turn a new leaf. To prove it, he re-branded himself 'Annatar' (Gift-bearer) and made powerful magical objects for all his friends called "rings of power". Each ring was actually a disguised way to control the kings of men, dwarves and elves. He had a master Ring that could control all the others, hopefully to turn all the people of Middle-Earth into his personal slaves. The elven lords manage to isolate their rings, realizing their intent. The dwarf kings were partially immune, so while they didn't become Sauron's servants, the dwarves did get an extreme lust for gold and extra greediness, which made the dwarves go into sort-of isolation while they tried to fix the messes this caused. Nine kings and sorcerers of men (including one woman, if you believe Iron Crown) were ensnared. They became Sauron's dark servants, the Ring Wraiths. Sauron meanwhile had got himself a rectangular Roman-style fort in Mordor, a valley almost subcontinental in scale.
On occasion the Númenóreans would hoist sail and head east to beat Sauron back. The Númenoreans were still mainly Loyal, in earlier outings. But on Sauron's final Second-Age incursion, the Númenóreans were led by Ar-Pharazôn, "The Golden". Yes this is a Semitic Phira'ûn reference, just like Quran. Tolkien was nothing if not a linguist. We are not told outright if Sauron's Nine were fighting alongside him yet; the two stories here, which we've conflated, don't inline with each other so well in Christopher Tolkien's edition. The sensible call is that he was holding them in reserve in case his upcoming epic plan failed.
By now both Númenórean factions owned colonies on Middle Earth. These clustered around the Bay of Belfalas on account of Mordor being the main threat: we know them as Gondor and Umbar. Also the Númenóreans had been enjoying a decent climate for the whole of the Second Age, and it was just nicer down there. Arnor, whence the Dúnedain originated, was neglected as too cold and non-strategic anymore. Ar-Pharazôn persecuted the Faithful at home but we're not told how far his writ ran in the Middle-Earth colonies; we assume that the Faithful drifted more toward Arnor, their ancestral home and with more Elves in it, because Third Age Umbar will end up a nest of Sauronism.
The Western fleet easily defeated Sauron's army. The Númenóreans all patted themselves on the back because capturing Sauron was totally their idea. Sauron's a Maia, so killing him is impossible, he just chilled in the dungeons, and told the Men how awesome they were even though they aren't elves... cause elves are immortal, hoping to give the Dúnedain lifespan envy so he could manipulate them with it. The Dúnedain started getting into fad diets and buying life-extension supplements on home-shopping channels, which in a sick twist of irony, made their lifespans shorter. "Well, if I was a Valar, I could make men essentially immortal," said Sauron, and the Dúnedain totally came up with the idea on their own to attack the Valar to demand immortality. "My old boss Melkor is a Vala, maybe he could help if he was free," and the Dúnedain fell for that bullshit too. Except for the Faithful but they couldn't talk sense into these idiots so they all took that western distraction to sneak off east for Gondor/Arnor.
So the Dúnedain prepared their armies, and sailed west. The Valar and Elves knew they were getting a full-on invasion, and appealed to Ilúvatar all-father to save their butts. Ilúvatar agreed and got his smiting on to decimate the invasion fleet, made Middle-Earth round instead of flat, and sank the island of Númenor like Thera for good measure. Sauron was still in it when the place was rekt. Some Men of Númenor managed to survive, mostly those who'd already colonized Middle-Earth. Those in Arnor were overwhelmingly Faithful; Gondor might have been a mix, but we can assume that the King's Men were predominant along the coast and what do you think happens to coasts when a continent is suddenly submerged offshore? (hint: it rhymes with "salami"). The survivors still had their abilities, and without the metropole they easily became KANGS in their own right, forming the majority in Gondor and an aristocratic minority in Arnor. Umbar, we dunno; they're not much of a factor in what follows, understandably, since their golden boy had died of extreme stupidity with his vizier to blame. They're more important in the Third Age.
Turns out Sauron too survived this "atalante" (get it??) because he still had his lich phylactery, the Ring; we don't know if he was wearing it when Shit Went Down or if he'd stashed it in the Barad-dûr. Either way, he came back, not as pretty as he once was, nor quite at his full power, but certainly in position to hit back at the Númenórean remnants in Middle Earth and at their allies starting with some border skirmishes.
The Elves (lead by their High King Gil-galad and the aforementioned Elrond) and the survivors of Númenor (lead by the new High King of Gondor and Arnor King Elendil) teamed up for a Pre Emptive Strike - and won! Although Elendil and his younger son Anarion bought the farm, his oldest son Isildur cut the Ring from Sauron and won the war. Just as the elves started thinking maybe not all Dúnedain were greedy shits like the ones that tried to attack Aman, Isildur decided the One Ring was too awesome to destroy and kept it for himself. Elves gave up on Men totally and with Gil-galad also dying in the battle and the High Kingship passing onto the Vanyar back in Aman took the Valar pardon and returned to Aman (in the case of the group who hadn't left Middle Earth in the first place, migrated) with some waiting out of guilt or honor (or simply attachment for their homes) until the Sauron situation was resolved. Isildur himself was later shot to death in an Orc ambush, losing the One Ring in a river. This also left Gondor and Arnor with two separate monarchies, not to be united until 3000 years later.
At some point five Maiar were sent as the Istari (Wizards) to try and stop even more stupid shit happening and make absolutely definitely certain that Sauron was absolutely definitely really dead this time. Two (
Alatar and Pallando Morinehtar and Rómestámo) went to the Eastern Kingdoms while three (Gandalf, Radaghast and Saruman) headed West.
Also Arnor managed to get themselves split into three kingdoms and then destroyed by the evil kingdom of Angmar that was lead by the most powerful Ringwraith, the Witch King. The Breelanders are the distant descendants of the last survivors. The Witch King was forced to retreat when Eärnur, 33rd King of Gondor and Glorfindel (who, despite dying in the escape from Gondolin's Fall taking a Balrog -as a side note, Ecthelion killed the Lord of Balrogs, Gothmog, in this battle at the cost of his own life- with him, was allowed to return to Middle Earth as a one off special allowance valid until the other lingering Elves depart to leave the Halls of Mandos where Elves who die are revived and stay) defeat him in the Battle of Fornost and destroy his army. Glorfindel makes the prophecy that the Witch King can not be killed by a man, making Eärnur let him escape. As a side note, a company of Hobbit archers were accompanying the Gondor, Arnor Survivor and Lindon-Rivendell Elf army. The last king of Gondor is then taunted about his sparing of the Witch King and decided to simply walk into Mordor and disappeared, with the direct line from Elendil ended, Gondor would be subsequently ruled by a line of Stewards.
At one point Gondor looked like they were getting buttfucked from all sides until a badass Northerner named Eorl the Young rode down and destroyed an army of Dunlenders, so the Steward of Gondor gave him a great wack of land to rule as King, founding the Kingdom of Rohan.
- The Tale of Beren and Lúthien - If you thought Aragorn and Arwen had a whirlwind romance, you ain't seen nothing yet. Lúthien's father King Thingol wouldn't allow her to marry Beren unless he captured one of the Silmarils from Morgoth's crown. Seemingly impossible demands aside, both of them go on an epic journey deep behind enemy lines to complete the quest. And though it costs them both their lives, they ultimately begat the line of the Half-elves after returning to Middle-Earth. Quite easily the most action-packed love story in high fantasy.
- The Children of Húrin - Húrin was among the mightiest of mortal men, and he alone defied Morgoth during the siege of Angband. When he was captured, Húrin refused to give up the location of Gondolin. As punishment, Morgoth imprisoned Húrin and laid a curse upon his children. Húrin's emo son Turin is forced to live on the run, and many times those close to him end up getting killed, mostly due to his overwhelming ego. He swings a big black sword that talks. When he meets an amnesic girl alone and naked in the woods, he does the most logical thing and marries her. Except, uh oh, he was tricked by a dragon into fucking and knocking up his sister so, of course, both an hero. If too much of this sounds like Elric that's because both Tolkien and Moorcock independently lifted from the same source, Kullervo (which Tolkien translated/retold alongside Beowulf). Welcome to Finland!
- The Fall of Gondolin - Gondolin is the last stronghold of the Noldor elves, hidden deep within the mountains, guided there by the Poseidon expy. Same expy sends the human warrior Tuor to warn Gondolin that it's about to fall. The elves don't pay much attention to his warning, but they let him marry the king's hot daughter. This pisses off her cousin, who's been wanting to get in on that incest action, and so he betrays the city and shows Morgoth's army how to sneak in. In the final battle, the city falls, but Tuor and his family escape, but not before shanking her cousin's treasonous ass.
Before they each got their Christopher treatment, an early version of Beren was had in "The Lay of Lúthien" (phnarr, phnarr) in an History of Middle Earth volume. The other two were had as Unfinished Tales.
The Rock Band
There's a bunch of Brits with guitars who formed a band called "Silmarillion" but the lawyers wouldn't let 'em, so they rebranded as "Marillion". Later they fired their lead singer.