Rats in the cellar is a common encounter or quest in game-oriented worlds. Heroes are tasked with dealing with a monster infestation in a cellar, sewers, cave, or abandoned location. While rats are common, other monsters may also be used, such as giant cockroaches, slimes, spiders, or other world-type specific creature. When designing these encounters, consider where the rats came from. There may be newly uncovered tunnels that lead to other subterranean locations like mines, crypts, sewers, or underground ruins. There may be a reason the rats have congregated and something else may be luring them there or controlling their actions.