A homebrew enemy my character dubbed reapers. Our dm said these things come from another plane and as they decay and age, they replace their bodies with technology, becoming undead construct magic using fighters will all the strengths and none of the weaknesses (IE, spells that affect undead don't effect them) in order to become perfection.

One of them was a monster called Duon, that looked basically like duriel from diablo 2, except mechanical. He was 2 bodies, one a caster, one a fighter, so fighting him was like fighting a level 14 caster and a level 14 fighter. He switched forms with a move action. That was the most horrifying monster i ever faced.

The most horrifying encounter i ever had (same campaign) was a Vampire Blackguard on a Beholder Mount (i'm not even freaking kidding about this). We won that fight, but died right after it (it was only the warmup, and we were already low on spells. We then proceeded to fight a group of vampires, 2 equivalents of Soul Edge (one of which could wield itself as a blue glowing figure). I was the highest level, a cleric3/wizard3/mystic theurge 9. And 3/4 of my spells were exhausted. Campaign ended after that.

What i PERSONALLY think is the most horrifying enemy is either a Chaos Beast or a Blue Slaad.