PokemonUnboundPokedex.com

Cinder Volcano (on B1F in the Shadow's storage area after using both Strength & Rock Smash)

Stealth Rock sets a hazard, causing damage to incoming Pokémon based on Rock's effectiveness; it doesn't stack, won't harm Magic Guard users, and can be removed by Rapid Spin or Defog. Damage varies from 3.125% (0.25× effective) to 50% (4× effective), with Castform taking 12.5% due to its form change.