It is the 51st Millennium and I cannot wake up from this nightmare! I cannot wake up!
— From the Introduction, and It Gets Worse from there.
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.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.
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).
God Is Evil: The Star Father, the aforementioned Chaos God of order.
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).
Last Stand: The Imperial Fists on Terra, the Orks on Octavius, the Space Wolves on Fenris, the Grey Knights and Custodes on Titan...
Private Military Contractor: The Free Companies (Astartes for hire), the Kroot (not suprisingly), Janisar's Fremen (no, notthoseFremen), and the Serf-Soldiery of Krieg (formerly the Death Korps of Krieg), among many others.
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 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.
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.
Truce Zone: Belius, the Barter King's world, free geld.
Ultimate Evil: The Nex, which is hinted to be related to the deepest layers of the Warp, and the Ophilim Kiasoz, of which almost nothing is known except for the above.