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.