Hive Rats

underdark. swarm of rats that gets smarter the more are together.
(maybe) a creation of the fomori post-dwarf. (maybe) an experiment of someone somewhere. leave it open like owlbears.
generate a gestalt sense of self. maybe even a mental projection. fights involve INT checks to perceive the swarm instead of illusionary bipeds.

Geographic Distribution

• The Underdark