Silverfish are small, skittish insectoids that make their homes inside stone and stone structures. Doing so offers them natural protection from the elements and predators, and allows them to fit natively into almost any environment.
They are only aggressive when disturbed, but they attack in swarms, and if any nearby Silverfish detect one of their brethren being harmed, they will jump out and assist.

They are quite weak individually, and often a glancing blow is enough to take one down - making even moderate sized swarms fairly manageable