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Baleful Swamp is an area of marshy land on Third Earth. It is a dreary and uninviting place, similar to the Living Ooze. Its waters are highly poisonous and the whole area is shrouded in a deadly mist. The source of Baleful Swamp is the River of Poison which drops as Dismal Falls before collecting as the swamp. Even the brave ThunderCats avoid venturing into the area of the Baleful Swamp and when they do, they wear the Thermal Catsuits to protect themselves from the harsh environment.
After Mumm-Ra armed the Mutants with the Totem of Dera, they set a trap for the ThunderCats at the Baleful Swamp. When Lion-O, WilyKit, and Snarfer arrived in the ThunderClaw, Vultureman used the Totem's power to bring to life a giant serpent from the swamp's waters which caused the three Cats to crash into the swamp. While trying to save his two friends, Lion-O accidentally swallowed some of the swamp's toxic water and became deathly ill. Later on Mumm-Ra used the Totem to create a monster of mud and sludge to attack the ThunderCats.