Nehrim/YMMV

Demonic Spiders: Those damn acolytes if you're a mage. With their reflect magic, and spell absorption, you often end up killing yourself with your own magic while they smack you around with their claymores.
- Also the actual Tarantulas. They're all over (one right outside the start of Part One!) - they're tough, spam paralytic spells so you spend more time on your face than your feet, and on top of it you don't even get to harvest any venom from them.
Game Breaker: The Magic Winds armour sets if you're a mage, with its massive magic regen bonus spamming high level magic attacks and backpedaling away from enemies makes the game pathetically easy.
Nightmare Fuel: Several.
- The giant spiders are moderately terrifying in large numbers.
- The old kingdom of the Dwarves. In fact, one part recreates the Tomb of Balin scene, but with a ton of zombies instead of goblins.
- When running away from the guards in the Southrealm Temple, you enter a cave. Inside the cave you come across this massive thing laying spider eggs. As you progress, suddenly there's a loud crashing sound and you realize that the thing is following you. You seem to escape it for a moment...Then it eats you. You survive. And no other reference is made to it.
That One Quest: The Professional. Oh god, The Professional.