Rats in the Cellar is a side quest in South Park: The Stick of Truth.
"Rats have infested the cellar beneath Skeeter's bar. Only a true hero could slay rats in the cellar."
When you enter the bar, you'll find Skeeter and his bartender hanging out in the empty space. Skeeter states it's because of the rat infestation in the cellar, which he offers to reward you for handling.
Taking his key and entering the decrepit dungeon, you'll find at least four giant rats meandering about the space. There are a few environmental hazards that can be used to remove some of the enemies in there, but you will have to fight at least one of them.
Once the threat is cleared, return to Skeeter and receive the reward of two new Facebook friends and Bar Darts as a ranged weapon.