Nightmare Fuel / The Roboutian Heresy

• Everything about the Raven Guard after their corruption. Even Daemons and Dark Eldar are nervous while dealing with them. Their corruption is so powerful that even most resilient Imperials are thoroughly tested before learning about their true nature. It's because their corruption comes from the bottom of the Warp, and originates from War in Heaven.
  • And everything about the life of Corvus Corax. His torment on Kiavahr is also a major Tear Jerker. Especially the fact he was led into corruption by Enemy Without of the very father he hates.
  • The Malice, a world of living flesh created accidentally from the results of several experiments of children of the raven and the crusading forces that tried to stop said experiments. Malice the planet is not to confused with the canon Malice, nor Malal.
  • They present themselves as followers of true Chaos... and there are multiple hints they're completely right. They can resurrect themselves, teleport through shadows from anywhere, and their corruption, as mentioned before, is so much worse than from anyone else.
• In a case of supreme irony, Roboute Guilliman shares his Father's fate, trapped on a life-sustaining throne, going from mad to sane then mad and back again. In order to free himself from this state of existence, he chooses to devour the souls of his own sons.
• The reveal of true nature of the Eldest, The Dragon to Fabius Bile, is. It's the being whose body belonged to Horus, and animated by Bile.
• Then we see what Enthronement means for the Emperor Himself. His struggle in the Warp is imprinted on the minds of all humanity, who, however, cannot comprehend His pained, endless scream: HELP ME.
• The fate of the members of the Emperor's Children who were taken from their ships during the fall of Chemos, remade and tortured by the resurrected Laer until they break into monsters known as "The Tithed Ones".
• The Dark Cells under the Emperor's Palace - a prison for multiple eldritch and humanoid abominations, including what appears to be a C'Tan, a xeno high priest of Malal, multiple human creations from the Dark Age, a sentient spear created by the Emperor, and many, many more.
  • One of the marines attacking the Palace remembered that they have built a time machine using a blueprint from the Dark Age of Technology to go to the past and prevent everything evil that happened ... which opened a portal to the future. A future, from which hideous, twisted beings that used to be humans whose screams make Astartes bleed and humans go crazy, started pouring. Only by succeeding on a very dangerous mission using teleportation, nukes and forbidden weapons did those marines manage to stop the invasion. Then they burned the planet they protected.
  • There is an implication that this future is what happened when Chaos had won and the Emperor didn't sacrifice himself... and which may still happen.
  • The fight ends with the Custodian in charge of this place using an ancient device to freeze this place in time for good, themselves included.
• The revelation of who the Yellow King is. It's the Emperor, or rather, his Shadow made from his rejected evil and terror and fear of his enemies, in addition to all the evil deeds made by Imperium. And Eisenhorn was groomed to be his avatar all this time. Successfully.
  • As it's the Emperor, it can take command of Space Marines (it happened to Kaldor Draigo, and he couldn't do anything to resist), and take their gifts. Imagine what it can do to the Imperium!
  • To make things worse. The first thing it does after securing its own existence via a Stable Time Loop is taking command of Malice, the living world itself. And the last thing we see said world doing is scream in terror.
  • The Voice, the Daemon who guided Raven's Guard to damnation? None other than Gideon Ravenor, who got twisted and changed by the Yellow King after his materialization.
  • And the last thing? He's coming for Corax and his legion. Consider that Corax has legion of Eldritch Abominations, has all the rest of features above, and will likely soon get an access to Akashic Records...
• The Ethereals are implied and now CONFIRMED to be at best pawns of Guilliman, and at worst his servants, subtly manipulating the Tau Empire into serving his designs, all but demonstrating that nothing and no one - not even the Tau - are truly safe from corruption by Chaos.