Ashaa is the leader of a group of aggressive, territorial rats that live under a parking complex outside of a mall occupied by much less hostile and much less territorial rats. They're all sort of covered in slime and they sort of smell like a combination of mildew and rot.
They may have a hive mind, but the party is so far unsure.
Ashaa is, however, capable of turning other creatures into his kind - which is where they think his numbers are coming from.