Warped rats can be found in the fifth room of the Lumbridge Catacombs Dungeon which can only be entered after completing The Blood Pact quest. They were created by Dragith Nurn, a now undead necromancer, during his experiments with local animals. This is one of the monsters that drop Mask part 2, which can be used with the 4 other parts to create the Mask of Dragith Nurn.