You call that a trap?

"A mob has been injured because of something you purposely left lying around the dungeon. From scattered Legos to spiked pits to buckets of flesh-devouring Skinner Ants to dimensional rifts that instantly boil all the blood in one’s body, the art of trapmaking has a celebrated and storied history in the annals of Dungeon Crawler World. So if you’re going to do it, you better do it right. Whatever it was that gave you this achievement, it was probably something stupid. Remember: If you don’t make it titillating, we will."