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.
- 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 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.