The Mutating Sprig is a tool added by Witchery. This is a more controlled, but limited, version of Mutandis or Mutandis Extremis. This tool is the only way to get a Leech Chest. Grass Blocks will mutate to Mycelium, underwater Dirt will mutate to Clay blocks, and vice versa.
Recipe as of 0.21.0:
Recipe prior to 0.21.0: