Time of Ending
The Time of Ending is one of the major time periods of the Imperium of Man. It was preceded by the Waning, stretching from mid-M41 to the end of M41. It was the "present day" of the 40k continuity right up until 8th edition.
The Time of Ending has seen the Tau emerge as a new power on the Eastern Fringe, Tyranids nomnomnom countless planets, and the Necrons begin to awaken from their sixty-million-year slumber. Abaddon is starting his 13th Black Crusade, an event that would surely be exciting had it not been covered in depth a decade ago. Lugft Huron rebelled in the Badab War and is becoming a major Chaos warlord himself. Rebellions are running like wildfire and the astropathic choir on Terra is constantly bombarded by pleas for help. The Adeptus Mechanicus has found an issue in the Golden Throne that they can't fix. More and more parts of the Imperium are being made the bitch of the Space Marines as planets, systems, and possibly even entire sub-sectors of the Imperium are given up for direct governance.
All in all, a bad time to be a High Lord of Terra.
Also called the Damocles Gulf Crusade, the Imperium tries to invade the nascent Tau Empire. At first, the campaign goes well but the Tau soon slow the crusade to a standstill. Given time the Imperium would decimate the Tau, but the First Tyrannic War begins, forcing the Imperium to call off the invasion. The Tau take note of their mistakes and start gearing their army towards facing the Imperium at large. The Imperium, being The Imperium, learns nothing.
First Tyrannic War
The Imperium's first encounter with the Tyranids, which saw the Ultramarines First Company get nommed down to a single Marine and an entire world of Ultramar stripped of all life. We know most of you
would be should be cheering the Tyranids on, but trust me, it's a bad thing and is in fact the reason that the only Ultramarines around to this day are such emo pricks.
The Siege of Vraks
As detailed in Imperial Armour Volumes Five, Six, and Seven, the munitions-stockpiling world of Vraks goes renegade under the guidance of the corrupt Cardinal Xaphan. The Death Korps of Krieg are called in to stem the rebellion. Seventeen years later, the siege finally ends with a full-scale Daemonic invasion and the intervention of the Grey Knights. Imperial forces "won", having spent countless resources and millions of lives to retake a now-worthless planet, polluted radioactively, chemically and biologically as well as ravaged by Nurglite plagues, with all the ammunition stockpiled on Vraks (the only thing worth fighting for) spent during the siege.
The Saint Cyllia Massacre
Massacre of Sanctuary 101
An Adepta Sororitas outpost is built upon a Tomb World, with the Sisters inadvertently wake up its sleeping inhabitants, giving the Ordo Xenos its first concrete confirmation of the existence of the Necrons.
The Badab War
As detailed in Imperial Armour Volumes Nine and Ten, Lugft Huron, Chapter Master of the Astral Claws, becomes too big for his britches and slowly builds an empire on the outskirts of the Maelstrom. He's eventually brought to battle after firing on Imperial vessels, leading to a massive, Space Marine-focused war. He's eventually defeated, flees into the Maelstrom, and starts heading the Red Corsairs.
Second War for Amageddon
Ghazhgkull Mag Uruk Thraka gathers one of the largest WAAAGH!s ever, laying siege to the vital hive world of Armageddon. Much badassery occurs, especially on the part of Commissar Yarrick.
Third War for Armageddon
The second war for Armageddon ended owing to Ghazhgkull getting bored and leaving. Long story short, he comes back with an even bigger WAAAGH! and attacks Hades Hive (the place where Yarrick held out in the last war) by nuking the site from orbit. Well, by dropping asteroids on it actually, but what the hell. He then proceeds to land on the planet and have some extra-large scale battles with the imperial forces on Armageddon. Commissar Yarrick returns to fight his old nemesis. This war hasn't ended yet, in case you haven't noticed.
13th Black Crusade
Cadia falls and a fucking gigantic Warp Storm cuts the Imperium in half. Craftworld Biel-Tan and Eldar (Oh wait, it's Aeldari now) culture in general is almost completely ruined by the premature birth of Ynnead, their god of the dead. Rhana Dandra draws nigh. Roboute Guilliman
returns from the dead wakes up from his nice long locked-in stasis due to a cut throat and poison nap and, needless to say, he's not very happy about the current state of the Imperium. The calendar turns 000.M42 (or does it?).
Thus begins the Age of the Dark Imperium.