"...It considered anything or anyone as suitable food and would devour the flesh, bones, and blood of its targets, leaving nothing behind. It was no more intelligent than most animals, and wasn't capable of making observations about its prey or whatever scenario it was in. It would find new prey by following strong magical signatures to their source, as living creatures generally had the highest concentration of magical energy that could be found...." - from the Re:Zero Wiki

I wanted something related to the idea of "Swarm"; but not necessarily a vermin like a swam of spiders, or rats - along with being recognizable to some more cultured individuals as a signal to start re-rolling at the top of the encounter.