What's worse than Warhammer 40k? Warhammer 50k. The Orks and Tyranids become a hybrid, The God Emperor has perished, the Imperium is shattered, the Necrons reawaken, Chaos reigns supreme, and that's just the beginning. The sheer amount of despair and horror can be unnerving at times, and the often Lovecraftian style of writing amplifies these elements. Basically, this is to 40k as 40k is to Star Wars or Star Trek. The story can be found here. Note that since it is posted on a forum, there will inevitably be some posts between the updates. There is now a sequel, The Age Of Dusk, which can be found here (parts 2-5 are here).Because this is a fanfiction of Warhammer 40,000, please consult that page for tropes pertaining to the original universe of which this fanfic is set.
Anticlimax: Ynnead, the Eldar God of Death which will rise up and finally defeat Slaanesh, comes into existence early. It's trapped in the Infinity Circuits, though.
Arson, Murder, and Jaywalking: On Captain Tastando Markus, who used his ship "to terrify isolated worlds into submission, or used his bloodthirsty crew to board ships, or invade particularly primitive worlds, plundering them of anything of value, raping anyone they chose to, and generally being unpleasant."
The Cavalry: A villainous example. Typhus the Traveller and his fleet arrive just in time (although it actually took them two weeks to traverse the Warp) to reinforce Abaddon's fleet during his war with Perturabo and Angron. The Nurglesque vessels besiege the Goliath Engine and heavily damage it, enough for Abaddon's capital ship to finish it off and win the battle.
Eldritch Abomination The Ophilim Kiasoz, which unmakes solar systems just by passing by them. Also the Star Father and the Nex. The New Devourer could possibly be this, but it's hard to know since it was Put on a Bus (it did eat about half the galaxy beforehand, though).
Framing Device: The story presents itself as a scholar's record to collect and store the history of the post-Imperial galaxy.
From Bad to Worse: Constantly. As the former Imperium crumbles, the scattered remnants of mankind grow more and more desperate before dying or falling to madness and brutality. The Tau are forced to grow harsher in order to survive the constant attacks on their worlds, and the Orks and Tyranids are wiped out to a man by the New Devourer save for a few survivors. Only Chaos and the Necrons gain anything: Chaos gets a new God and takes over huge chunks of Imperial space - including Cadia and Terra - and the Necrons revive the Void Dragon and go on a rampage throughout the universe.
God Is Evil: The Star Father, the aforementioned Chaos God of order.
While the Star Father is technically a chaos god, there are distinctive differences between him and the others- one of the most notable being that Angyl worlds (His version of Daemon worlds) have a weakened connection with the Warp, opposed to a stronger one in the case of the other chaos gods. Also his Catch Phrase: "OBEY!"
Hidden Elf Village: Many Forge Worlds become this, often combining it with Mordor. Ironically averted with the surviving Eldar, who manage to trick their way into top positions of several Petty Imperiums (the Exodites), become pirates (the Corsairs), or become The Empire (Biel-Tan).
Knight in Sour Armor: What happened to the Tau in wake of the grimness of the endless destruction rampant throughout the galaxy. Though they don't wipe out any race they encounter like the Imperium, their diplomatic policy has changed from "negotiate first, then subdue if necessary" to simply subdue.
Last Stand: The Imperial Fists on Terra, the Orks on Octarius, the Space Wolves on Fenris, the Grey Knights and Custodes on Titan...
The Reveal: At the end, it is revealed that the (in-universe) author of the Fictional Document is none other than Inquisitor Kryptmann, the person who first "discovered" the Tyranids. He joined the Order of the Recollectors in order to create a summary of post-Imperial history, which is the story itself, as mentioned above. In addition, the Hermit mentioned is in fact Vulkan, Primarch of the Salamanders, who is building an empire centered off Armageddon.
Scavenger World: Even more prevalent than in 40k. Gerhed Lussor's Rogue Trader Empire is an entire star empire based on scavenging.