Rage (?) Drakes, level 2 critters that do +5 damage when there are two or more of them.
If the terrain isn't too cluttered and/or they start close (so the PCs can't just ventilate them) they do terrible, terrible damage.
I've used them twice. First time they killd two PCs. Killed! In 4e, it's easy to knock a PC out but not easy to kill them. They didn't chew on the unconscious PCs either, they outright killed them. (We lost both the warlord and the cleric that battle. Ow.)
Second time the terrain was much nicer for the PCs. The passageways were too narrow for the drakes to gang up on the PCs, and so the darkes were ventilated by arrows.