Helm: Mushroom Crown

Helm: Mushroom Head

Armour: Mushroom Body

Gauntlet: Mushroom Arms

Leggings: Mushroom Legs

Where To Find: 

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The Mushroom Crown can be found on a corpse hanging from a pillar ruin in the southeastern corner of Lake of Rot.

As for the other pieces of the set, they are located in Seethewater Cave within a cavern inhabited by a Giant Poison Flower and several mushroom people.