Unlike the other armour sets mentioned so far, the Cleanrot Set is dropped piece-by-piece as random loot from Cleanrot Knights in the Caelid Wilds. The two best places to farm Cleanrot Knights are Aeonia Swamp in the centre of Caelid, and the War-Dead Catacombs - a dungeon that can only be accessed from the battlefield where you fight Starscourge Radahn.