Age of the Dark Imperium
The Age of the Dark Imperium is one of the major time periods of the Imperium of Man. It was preceded by the Time of Ending, starting in M42. As of Warhammer 40,000 8th edition, it is the "present day" of the setting.
Despite the Imperium being cut in half by the biggest Warp Storm the galaxy has ever seen, the Imperium has rallied around the resurrected Roboute Guilliman, now back in his old role as Lord Commander of the Imperium. While the Primarch has managed to fix a substantial deal of the damage caused to the Imperium by the Great Rift, even he knows that the worst is yet to come.
Main article: Great Rift
Also known as the Blackness in Low Gothic, the Noctis Aeterna was an event caused by the formation of the Great Rift where the light of the Astronomican vanished and Daemonic incursions ran rampant. The event appeared to distort time itself, lasting a few days on some planets, while on Terra it lasted for just over a month. (And it's speculated that it's still going on for some unfortunate planets.) Judging by the time-frame presented in Watchers of the Throne: the Emperor's Legion, the Noctis Aeterna began a few months after the Fall of Cadia and partway through Roboute Guilliman's voyage to return to Terra, although the aforementioned time distortion means it is only an accurate timeline from Terra's perspective. From Baal's perspective, it only lasted a day or two, yet when it cleared up the Blood Angels had discovered that seventy years had passed in the outside galaxy. The event culminated in an army of Daemons of Khorne invading Terra itself. While this was repulsed thanks to Guilliman and the Custodes, the attack still shook the Imperium to its core as it proved that the influence of Chaos was stronger than ever before and that even the Throneworld was no longer safe from its reach.
The consequences of this left major wounds that have still yet to heal. The Imperium lost contact with half of all known Space Marine Chapters, and twelve chapter homeworlds were confirmed to be destroyed. Every Eldar Craftworld was assaulted by Chaos forces (including the ones hidden in the Webway), and they have yet to account for the fate of two of them. Armageddon was transformed into a half-Daemon World, turning the war between Orks and humans into a four-way clusterfuck as Daemons of Khorne and Tzeentch joined in on the fun. On the positive side, the Blackness unintentionally managed to save the Blood Angels, as it allowed Ka'Bandha to destroy the tendril of Hive Fleet Leviathan attacking Baal as the Great Rift dragged many of the Hive Fleet's bioships into the Warp.
From this point forward, attempts at forming an objective chronology become increasingly pointless due to the continuing time distortions across the galaxy; most worlds end up using the formation of the Great Rift as their new "Year Zero" as a result, conveniently timed at 000.M42 thus keeping the calendar somewhat intact for out of universe purposes.
The Terran Crusade were events surrounding the newly-resurrected Roboute Guilliman. After going around Ultima Segmentum and securing his holdings from further Chaos incursions: Guilliman sought to return to Holy Terra to have an audience with the Emperor. Naturally, Chaos did everything to keep him from returning, from Fulgrim attempting to subtly corrupt him into Slaanesh, to Nurgle trying to keep him anchored in the sector by making a plague that can only be healed by being admitted into Guilliman's presence. With a steel will and heavy heart however, he couldn't be swayed, and he got a fleet of whatever Imperial forces he could muster and started his pilgrimage back to Terra. Again, naturally Chaos was not having any of that shit.
The trip was incredibly perilous, the Dark Gods made warp travel incredibly turbulent that it heavily stalled their pilgrimage, including raids by the Thousand Sons and Red Corsairs. Eventually, after making several desperate blind warp jumps and intense fighting: the crusade was ultimately captured after Kairos and crew boarded the Macragge's Honor, dueled Guilliman and captured him with some subtle warpfuckery. On the threat of having their spiritual liege killed: the rest of the crusaders surrendered and were taken prisoner.
After that, some other convenient things happened. Guilliman and crew were being imprisoned in a Blackstone Fortress owned by Huron Blackheart, while Karios and Skarbrand were having an argument, which turned sour. Kairos wanted to keep Guilliman alive for future plans, while Skarbrand wanted him dead. Being offended by the lack of consensus, Skarbrand and his warband invaded the Blackstone Fortress to kill Guilliman. During the fighting, Cypher and his Fallen Angels, broke into the fortress, released the primarch and his troops, and helped them escape, on the condition that Cypher be allowed to see the Emperor, which Guilliman reluctantly agreed to.
After that, the Eldar intervened and led the party to a webway gate to allow them to escape the fortress. Seeing this, the Khornate and Tzeentchian warbands temporarily put aside their differences, as their prize is escaping. Skarbrand saw Guilliman making a break for the webway gate and charged at him with furious resolve after decrying his hatred for the color blue, which coincidentally is the color of Guilliman's armor and the webway itself. Things were bleak and Guilliman was only able to escape after Black Templar Marshal Amalrich managed to distract Skarbrand long enough by sacrificing himself to impale Skarbrand with a sword. Distracted by the sword in his gut, Skarbrand took a moment to bellow his hatred for swords, before having his head bisected by Guilliman.
Things didn't let up though, as the Thousand Sons led by Magnus himself was on the chase. Not wanting to potentially lead the Crimson Cyclops into Terra: Guilliman asked the Eldar to lead them onto Luna instead, which they escaped to, but not before Magnus and crew managed to follow them through the webway, as the Eldar couldn't seal the webway gate in time. Things were looking bad once again for the crusaders, until a Terran response force comprised of Battlefleet Solar (the fleet incharge of guarding Holy Terra itself), Imperial Fists, Sisters of Silence, and the The Emperor's Golden Banana Men, appeared on the scene. The combination of the Sisters of Silence's psychic null field and the collective might of the forces brought together, were enough to beat back Magnus and his cohorts back into the Warp.
Finally safe after so many sacrifices: Guilliman was finally granted an audience with the Emperor. As for the rest: the Eldar disappeared as mysteriously as they arrived, and Cypher and his Fallen Angels were arrested after Guilliman was horrified by the sword Cypher was carrying, and could not allow him to get near The Emperor. Naturally, Cypher escaped captivity a few hours later, but was noticeably miffed by the betrayal.
What him and The Emperor discussed is a mystery, but Guilliman went to the Senatorum Imperialis, told the High Lords of Terra how bad they were and how he was disappointed at how they ruined The Emperor's vision for humanity, declared himself Lord Commander of the Imperium (again), effectively having political power second only to the Emperor himself. (At least in theory. The reality is that the Imperium's 10,000 years of unchecked bureaucracy combined with all the power-hungry assholes in the Adeptus Terra mean that even Guilliman's position can't save him from the Chaos God of Red Tape bogging him down. The High Lords even planned a coup to depose him, which was stopped only when the Custodes threatened to intervene.) Having assumed his new role, he set about preparing for the dark times the Imperium had ahead.
Big G's first military campaign after crowning himself the Lord Commander of the Imperium. The Indomitus Crusade was aimed at shifting the Imperium into the offensive after the damage done by the 13th Black Crusade by going into the stranded "Dark Imperium" and try to liberate as many besieged Imperial worlds as possible, while exterminatusing those unable to be taken. Additionally, the other objective is simply to kick Chaos out of their lawn and try to re-establish some form of order in the Imperium so Guilliman can begin to fix what's left of it.
By the end of the Crusade, Chaos' momentum had effectively been destabilized and halted, but the damage has been done. Hundreds of Imperial worlds have either been destroyed or corrupted beyond measure and while Chaos' advance has been stalled for the moment they're still causing trouble for the Imperials. On top of that, much of the "Dark Imperium" is still cut off from the Astronomican and is generally defenseless.
Overall, Guilliman stopped the Imperium from bleeding to death, but now he has to contend with a nasty infection from the wound. Still, you can nurse a sick man back to health, but you can't do anything anymore once he's dead.
Speaking of infections and sickness, during the confusion Nurgle's armies invade Ultramar, forcing Guilliman to return to defend his home system.
Cawl's Primaris Marines are used to reinforce a number of Marine Chapters to get them back into optimal fighting strength, while some new Chapters composed entirely of Primaris Marines are also founded.
While the majority of Chapters have welcomed or at least accepted the reinforcements, there is still a considerable deal of distrust among Chapters who are wary of the newcomers, as well as those who fear that the "Firstborn" Space Marines may ultimately be replaced. For now at least, these concerns have been left on the back burner in favor of dealing with more immediate problems.
Nurgle's offensive into Ultramar. Using the Scourge Stars as a staging area, Mortarion led Nurgle armies in a bloody campaign to annex Ultramar into Nurgle's realm. The home team is led by none other than Grandpappy Smurf himself (also a reason he had to cut the Indomitus Crusade short).
The war was decided in The Battle for Konor, which saw Mortarion being pushed back but at a great cost; dozens of worlds were either destroyed or corrupted beyond salvation. Morty would have probably went in for another go, but because most of Nurgle's forces were sent on his campaign the other Chaos Gods decided to invade the Scourge Stars for shits and giggles. As a result, the Plaguefather had to call Mortarion back to deal with his rivals. In light of the new threats that they face, Guilliman charges its new Tetrarchs with expanding Ultramar's borders, the better for pushing back any future attacks.
Magnus then one-upped Mortarion by taking Stygius Sector.
War of Beasts
Centered on the world of Vigilus, it's more or less the Imperium's desperate attempt to keep a vital foothold in the Dark Imperium in Mankind's hands, and in many ways is like the clusterfuck at the Kaurava System but with actual stakes.
Initially it was just a campaign to repel an unexpected Ork invasion, but soon exploded into a massive free-for-all when a Genestealer Cult-led rebellion threatened to undermine the planet's defenses at the same time. What's more, Dark Eldar raiders stalk the underhive, while Chaos-tainted madmen and the Death Guard make their presence known all-too soon. And on top of that, a misunderstanding with the Eldar of Saim-Hann provokes them into performing their own attacks against the Inperials. The situation went from bad to awful when Abaddon paid a personal visit, along with a metric fuckton of Chaos Space Marines.
The Imperials under Marneus Calgar were just barely able to hold Vigilus through the ensuing clusterfuck, but only after taking heavy losses themselves; Calgar himself was nearly slain by Abaddon in a duel with the Warmaster.
The campaign has been the focus of several boxed sets (Speed Freekz, Tooth and Claw, Wake the Dead, Urban Conquest), and described in full in the Vigilus Defiant and Vigilus Ablaze campaign supplements.)