IT IS THE 42nd millennium. The Tyranids have eaten Ultramar, the proud Ultramarines are presumed killed to the last man with a strike force lost in the warp. The commander of the Word Bearers sent a message of condolence to the High Lords of Terra, then curbstomped Cadia. Isolated fragments of the Cadian Guard and Titan units still hold out in the mountains, and some regiments were off-world. The Word Bearer's message was intercepted by Clade Vanus of the Officio Assassinorum but opened a warp-rift that destroyed 3 sub-levels of a hive. Clade Vanus was blamed, and a small war ensued for 13 months between Clade Vindicare, Clade Vanus and an Astartes garrison force. 5,000 citizens starved to death, too afraid to go outside during the war. One hive governor was shot at dinner, the rest of the guests were too scared to move and carried on dining around his corpse.
A Hive Fleet reached the doors to the Imperial Palace but was repelled, having taken down Earth's defenses with assimilated Eldar biotechnology and slaves. Ghostly apparitions are manifesting in the Imperial Palace. Necrons have all but conquered a sector in Segmentum Ultimus, Imperial Forces tactically advanced in the opposite direction while the Necrons were backdoor pummeled by Eldar, Orks and Eldar Renegades, the latter of which are lead by "a mysterious figure." Several High Lords of Terra flee to Maginor, face death by Orks, who ignore them in favour of attacking another Waaagh.
The Tau accidentally invade the Webway, and several large Tau fleets are now infested with Harlequins. Two never before seen Space Marine chapters appear and wreak havoc across multiple segments. Their chapter names are not confirmed, but it's along the lines of "Templar's Death Goth Angels." One Chapter Master, whose speech is near un-intelligble, has been lost in the Webway for millennia, doomed to slaughter and re-slaughter immortal Eldar spirits as well as lost Tau. He is sometimes able to surf a Tau ship into real-space but is forced back into the Webway by his honour oath to aid Ensign Amethystra Star Shinobi Aiko Pikard Janemay Sue, the troubled Medical Officer/Secret Inquisitor of the flagship Gloria Imperialis.
To sum up: Tyranids eat Ultramar and land on Earth. Necrons conquer a sector, and are in turn re-conquered. There is a mysterious figure. The Tau are lost in an Eldar bus terminal. Space Marines do their thing, with bad accents. The Orks go Waaaaaagh! The Blood Angels have not responded to questions about Necrons, other than to say "butt out."
In the grim dark darkness of the far grimness, there is only future, and it is grim. To be alive in such times is to have no clue what's going on, and you'll probably die before you're 40. With a headache.