Contract: Swamp Thing is a witcher contract in Crookback Bog, Velen. A mysterious fog that kills everyone who enters it has appeared near the village of Downwarren, right where the locals dig peat. Geralt can start the contract from the notice board, by finding the fog himself, or by meeting Leslav, a sick peat digger who warns him about the strangling mist.
The fog is the work of Ignis Fatuus, an ancient foglet that has lived in a cave in the bog for a very long time and grown stronger with every year. Foglets hide in the mist, create illusions of themselves and strike from behind, and this one is a master of the art.
Moon Dust bombs to stop it from regenerating, Quen or heavy armour for its brutal blows, and striking the illusions to reveal the real creature are the keys to winning. Stepping out of the fog to recover also helps, because Ignis Fatuus does not like to leave it. It is one of the most atmospheric fights in the game.