- The truly horrifying thing about the Horus Heresy is actually Fridge Horror. The way the Imperium of Man is set up as it is in the 41st millennium was intentionally designed to prevent another Civil War on the scale of the Horus Heresy. In other words, the Imperium of Man is a xenophobic, dictatorial, theocratic hellhole to make sure something like the Horus Heresy can never happen again. What truly leaves one terrified as that as you read the series, you realize that they were right to do this.
- The virus bombing of Isstvan III. We get a really detailed description of just how the Life-Eater virus works, and it's scary: flesh begins sloughing off within seconds, nervous systems collapse, and bones dissolve into jelly. The virus moves so fast it practically travels at the speed of thought.
- Every single time daemonic possession comes up. Effects range from simple transformation into a bloated corpse to the host's body turning inside out.
- Everything about the Slaaneshi corruption of the Emperor's Children in Fulgrim. Fabius Bile enhances them physically and driven by the insanity of the Laer Temple, their ship becomes host to a ton of nightmarishly debauched art, with Fulgrim's own chamber becoming host to what one narrator describes as 'beyond pornographic' artwork. This isn't even going into the Maraviglia, a debased concert which also happens to basically be a Slaaneshi ritual, resulting in such delights as audience members gleefully beating others to death then turning back to the music completely unconcerned, the dancers killing themselves in performing completely inhuman movements, the singers doing the same while capturing notes human throats aren't meant to sing, the summoning of a group of Daemonettes who kill a good chunk of the audience who are too enraptured to care, and finally the rampage of the first Noise Marines, who turn the concert's special instruments into weapons which tear their victims apart with waves of sound. By the time of Isstvan V, the Legion has degenerated from proud but noble warriors into a horde of twisted hedonists, and everything about their fall is nightmarish.
- Serena d'Angelus is perhaps the worst part. Originally introduced as a fairly kind and gentle painter, she is so twisted by Slaaneshi corruption and her pre-existing low self-esteem that she seduces a fellow remembrancer for the sole purpose of luring him back to her room so she can kill him and use his blood and body as materials for her paintings. And she does this multiple times.
- The Emperor's Children get, if anything, worse by the time of The Damnation of Pythos. Iron Hands troops boarding one of their ships find, among other things, a meticulously crafted carpet made of human tissues and a door with human bodies embedded in it, apparently still alive when they were immersed in the molten metal.
- Vulkan's torture at Night Haunter's hands. He's forced to kill his sons, fight his own loyalist brother, watch people around him die, has his hopes raised only for Curze to crush them and all the while, visions of Ferrus, blaming him for his death, plague him. Worse, he can't escape this by dying, because he can't die, and when he finally manages to escape, he's insane to the point that nothing but desire for Curze's death remains of a man who was once the one White Sheep of the Primarchs.
- The savagery of the Word Bearers as they settle the score with the Ultramarines at Calth is quite something. While the XVII Legion were apparently notorious for excessive violence during the Great Crusade, it pales in comparison to what they do in the name of getting even with those they hold responsible for their humiliation. Consider the below example, taken from the audio drama Garro: Oath of Moment as an example of the depths of brutality the Word Bearers sink to in the name of revenge:
"He saw a group of Word Bearers kill an Ultramarine with a hurricane of storm bolters. To his disgust, even when the Astartes fell dead to the ground, they continued to fire into his body, unloading round after round into the twitching mess of the corpse...and as they did so, he heard them laughing".
- The line below from the same so succicently sums up the fall of the Word Bearers (and the last part could be interpreted as referring to all the Traitor Legions) to Chaos that it sent a chill down this troper's spine:
"The Master of Mankind had personally rebuked the Primarch Lorgar, chastising him for fostering idolatry of his father and violence beyond the pale. Some believed Lorgar and his legion had heeded these lessons, but now it seemed clear that, if anything, they had rejected their lord and found a new path, one of cruelty and carnage, fuelled by raw hate and new gods".
- The description of the bio-weapons used by the Death Guard as they attack the White Scars at Catullus with the full intention of exterminating the Vth Legion to stop them interfering in Horus's plans any further are horrific. Even more worrying is the possibility that given Mortarion's increasing dabblings with the Warp since Scars, these bombs could be less man-made weapons and more something crafted with the assistance of Nurgle.
"Hundreds of the bombs exploded, flooding the frigate's lower decks with boiling green fogs that churned and hissed their way through solid adamantium. The crew, even those in protective armour, were eaten alive, their atmosphere filters blown and their eye-masks fizzing.
- The description of the Keeper of Secrets Maushya-Rakshasi and its Daemonette minions possessing the crew of the Emperor's Children warship Ravisher to manifest in the material universe is horrific. The Space Marines becoming daemonhosts split open and come apart as the daemons tear their way into existence through their host bodies.
" His outer battle-plate exploded outwards, revealing a new, sinuous underbody of violet flesh and black veins. His stature swelled to obscene portions until he towered over them. Arms, four of them, corded and bound with leather, unravelled in place of his own, and horns erupted, twisting, from the bones of his dissolving face. He took a first step, and instead of a heavy-tread boot, a cloven hoof cracked the deck".
- Even more unsettling, as the transformation begins, the Ravisher's bridge crew go for weapons and try to put the possessed Astartes down. Against Slaaneshi daemons, this is the worst thing they could have done, as the pain of being hit and shot does nothing but accelerate the transformations.
Nightmare Fuel / Horus Heresy
The Horus Heresy began when Horus Lupercal turned from his Father's light and embraced Chaos. Each novel in the series has plenty of horrors to be unleashed (or has been unleashed), and it only gets worse book by book.