The series follows the adventures of the protagonist Guts, who does battle against the forces of darkness with a really, REALLY big sword. In fact, it's probably the only anime/manga in existence where the other characters actually notice that someone's sword is unrealistically large, often pointing out "Fuck me, that's a big swoOH GOD IT CUT ME IN HALF BLARGH". It is, in fact, so large that it cannot actually be used while mounted because swinging it makes the horse fall over. Guts himself can only use it by virtue of training with man-sized swords since he was a small child and scaling upwards as he grew, in combination with his titanic (and possibly supernatural) strength.
Chronologically, the story's beginnings are simple gritty medieval violence, following the exploits of a band of mercenaries as they make a name for themselves in the world and acquire power and glory. It swiftly descends into Oh-God-What-the-Fuck-Horror when Eldritch Abominations from Hell and their monstrous "apostles" get in on the action, and via a roundabout series of events Guts ends up hunting down his old companion now ascended to godhood, determined to get revenge for his betrayal.
Just to add to the grimdarkness he has a waifu that got raped by his best friend who'd turned into a demon. His waifu then had a miscarriage from all the trauma and his kid's fetus comes back to haunt him every day. Later in the series his kid comes back looking about the age of 3-8 years old and saves him and his friends by using terrifyingly effective abilities on par with lofn. Then said kid disappears again leaving a puzzled Guts and a slightly distressed but still insane waifu.
The series has a massive following both in Japan and in the west and has been running for almost two decades. The author, Miura, is well known for taking extended breaks from production, and given his advancing age and his recent statement that he considers the series about two-thirds complete, has led to fears by many that he will kick the bucket long before he actually finishes the series.
/tg/ greatly enjoys it because it's essentially a fucked-up mix of Warhammer 40000, WARMACHINE, D&D 3.5 and Exalted. Furthermore, Guts does display some vestige of humanity, despite all the traumatizing shit that's happened to him he still displays empathy for other people. Where thugs, corrupt guards, and thieves all get chopped to bits in seconds, Guts at least attempts to avoid killing innocents. Insofar as it wont hurt him. For the most part he is an amoral badass/cuntbag, yet there's still a bit of a caring dude in there. Somewhere.
Our favorite protagonist. Fights with a FUCKHUEG sword, eat your heart out Cloud. He is as badass as Eliphas and as manly as Kenshiro. Has an impossibly depressing past that began as he was born... prematurely, from the corpse of his hanged mother who'd been strung up by bandits, likely only surviving the fall due to the soft mud from the rain. From there he lost his kindly adoptive mother to a plague, fought as a child soldier, got sold as a sex slave by his asshole of an adoptive father... you get the idea. He also got raped (like nearly everyone else in the story). When he first encounters Griffith, he loses to him in a sword battle (and his fuckhuge sword manages to lose to a rapier. What.) (Happens Because Griffith is the first enemy to use actual tactics beyond "throw yourself against the FUCKHUEG sword-wielding sociopath and hope you win")., and then goes on to fight with Griffith's band of mercenaries. Things happen, he gets the hots for Casca, then all his life went to shit with the Eclipse, where his waifu got raped by his former best friend while he was forced to watch, and lost his left forearm and right eye. Then he gets new companions and things become a bit less grimdark.
Token pretty boy. Very, very gay for Guts. Still a sword-fighting badass and a Creed-level general. Leads the Band of the Hawk, a mercenary group. His goal is ambition: then he manages to fuck it up (literally) by boning his king's daughter. The king (who also wants to fuck his daughter) goes to torture him horrifically, and he is broken. After that he goes to sacrifice the whole Band of the Hawk (except some kid, and Guts/Casca who manage to survive) to the local equivalent of the Chaos Gods and he becomes a god. By raping Casca. And it is shown in detail, eughhh. Then he goes all mary-sue by going to lead Midland, still under the appearance of Griffith and the local inhabitants fail to get anything. He then transforms the world into a Daemon world save for the FUCKHUEG city of bling he built himself, in which all of humanity (those who survived at least) are hidden away from the Daemons that roam the land; as any good inquisitor would say, this is highly fucking suspicious: could he be gathering victims for another more massive sacrifice? More like his trying to become the God Emperor of mankind everyone there is worshiping him as their god. Also, the guy who did nothing wrong.
Delicious chocolate skin short-haired tomboy warrior. What's not to love?
Guts's love interest and the greatest rape-bait in all the manga. Seriously, try to count how many times the girl got rape threats... Still, a pretty badass fighter before the Eclipse and Griffith's second in command. Pretty tsundere for Guts. Then the Eclipse (and rape) happens, and she becomes insane. After the Eclipse, she becomes Guts's protégé, as he really wants to fu- for her to love him again and Guts seeks a cure for her. UPDATE : He's done it. It only took more than an actual decade, no big deal.
A Khornate Bloodthirster, except oldschool and honorable. He sought to become an Apostle because he wanted to continue to fight and kill. Still murders lots of people but doesn't have any fun unless they can fight back. He seems to like Guts as an enemy, and even Guts tolerates him more than the other apostles (even though they're trying to kill each other). Also is tsundere for the Skull Knight because they have fought for centuries. Equal of Guts in combat and generally pretty damn bad-ass. Doesn't even need weapons in Bloodthirster-mode, in fact both times he's used against Guts was because the fights broke his sword. Has only outright lost to the Skull Knight and Griffith.
The Skull Knight
A bad-ass undead knight with a skeleton's head. He is actually a good guy, despite being just as evil looking as the monsters he fights, as he saved Guts and Casca from the Eclipse. Zodd's rival since a few hundred years. Has a pretty impressive sword, two of them in fact, though he is more sensible than Guts as his is not FUCKHUEG and he carries a shield and it is heavily implied throughout the manga that he is King Gaiseric. (Even though a longsword isn't the best weapon on horseback, who cares, he looks awesome anyway.) He is also the undead embodiment of Deus ex Machina, seeing how many times he shows up to save a character and then promptly disappear.
FIGHTING - 10: Actual varied fighting styles and the main character waving a fuckhuge sword around. Then you've got the monster fights.
GURO - 9001: As one youtuber put it, "GORE!!! GORE!!! BLOOD, TITS GORE!!! GIANT ASS SWORD!!! YOU SEE THAT DEMON? WELL NO YOU DON'T BECAUSE IT'S FUCKING DEAD!!! AGH BLOOD GORE!!! AAAAUUUGHHH!!!" To be more specific you get to see people and creatures get dismembered and mutilated in all kind of fun ways.
Gayness - 9001: As is mandatory for all animu reviewed by /tg/. You've got gay rapists everywhere, and pedophiles abound. Guts was even raped by one as a child, although he killed him later. And then there's Griffith, who, while he isn't gay himself, appears so effeminate it's impossible not to think this. Also he fucks a rich old guy to fund his mercenary band, not to mention the movies have him thinking of Guts while he fucks a hot woman.
Moralfaggotry - 0.00001: Unbelievably Grimdark, and miraculously is not Edgy. Guts is shown almost without any form of moral compass, and it is glorious. Oh, a bunch of innocent little children got turned into monsters through no fault of their own? Don't care, kill 'em all and use their bodily fluids to make myself fireproof. An innocent man being beheaded to get me to come out of hiding? Watch and laugh while he dies. Wound an apostle, fill him full of crossbow bolts... slowly, and then drop a burning building on him and laugh while he burns to death? You bet! However, that's all only on the surface. Guts only acts like an asshole because the last people he cared about all got horribly killed, and most of those things are done out of pure pragmatism rather than maliciousness, since he doesn't murder people with no reason, the fairy children themselves bordered on eldritch abominations. Later he gets some new friends and lightens up to place the morality more at a 5 or 8.
Lulz - 10: So many innocents die in this series, in truly hilarious manners, from completely unneeded torture by the Inquisition to mass-murder-orgy-fuckfest-massacres done by chaos-worshipping pagans. So do many villains. So do many random background people, as well as inanimate objects and plants. Even main characters aren't safe. This only begs the question: how many people still live in Midland?
Rape - 9999: Oh God, OH GOD with the rape! If there's one thing that Miura has drilled in our heads for damn sure, it's that nobody is safe from rape in this series. In fact, rape is so prevalent that you can even consider it a theme. Hell, Guts' waifu is such rape bait that three out of five guys are going to try to rape her. Most monsters will try to rape you, most people will try to rape you. In general, most things will try to rape most other things. Even Rape Horse? Especially Rape Horse.
Sickfuckery - 9001: The kingdom has a mercenary band of demonic men who march with impaled nude women as banners. You read that right. Also, remember the Daemonculaba? Guess what, someone in the Berserk Verse had a similar idea. Take a pregnant woman, dunk her inside the womb of a giant womb-demon, cue demon baby clawing its way out of her stomach so it can quickly grow into a shock trooper for the army. Repeat with entire city population.
MIURA IS ALIIIVEEEE
The author, having realized it's too boring to continue the story with only Guts, his tinkle fairy and his mentally ill waifu, decided to "soften" up the manga a little bit by introducing a loli witch name Schierke and another tinkle fairy from Peter Pan. This causes rage among the fanbase, since they believe Berserk is no longer about a manly, badass mercenary with his vengeance against his homo-looking former comrade, but instead adding side stories about the loli and the shota, even making Schierke naked in front of Guts for some fan service. But since we have seen mutated children rape each other to death with wasp stingers and countless instances of rape and Casca tentacle porn, we can say the author is also a rule 34 fanboy. Besides, it's not like Guts lost his manliness anyway, since he's still the only one who gets shit done most of the time.
The manga is shifting its tone from low fantasy grim dark to high fantasy Nobledark over time. This is made obvious by the tone shift after Puck reintroduction, Peekaf shit, Tower of Doom and culminating in him acquiring the broken-as-shit Chaos-tainted Power Armor. He slowly opens his heart to his new comrades, possibly ending the story in DBZesque 6-volume fighting scene. Basically Miura is moving up from Riddle of Steel, to oWoD, DnD 3.5 and finally finished in Exalted (presumably so Guts can stand a chance in the penultimate fight against the Godhand).
It should be noted that Miura is a long time fan of shoujo (manga and anime aimed for girls) and his writing and art often uses the shoujo style so this really isn't a surprise to anyone.