This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Raidou Kuzunoha vs. King Abaddon
There is a fair share of creepy things in general, but what really gets to some people is the White Rider's horse, which is covered in eyes. (By Body Horror standards that's really pretty tame, but eyes appearing where they shouldn't causes a strong reaction in a lot of people, not unlike the infamous Lotusboob photoshop. And if you don't know what that is, good for you.)
Once the Apollyon descend upon the Capital, just about everything gets much more horrific.