“Rats. We’re Rats. We’re the Rats. We prey at night. We stalk at night. We’re the Rats.” 

On each plane of existence, there exists one great Rat who rules over all other rats on that plane. This Rat is known as The Giant Rat That Makes All of the Rules. There is always one of these Rats on each plane at any given time and when it dies, a new one takes its place. The Rat doesn’t posses any extraordinary or supernatural abilities besides the ability to speak Common. Despite this, all other rats follow its commands. 

This Rat is known for getting into trouble. Whenever it spots an opportunity to disrupt some plan or cause mischief, it takes it.

(Based on this great video) 

(version 2)