If held by an Swampert, allows it to Mega Evolve into Mega Swampert if its Trainer has a Key Stone. Each Trainer can only Mega Evolve one Pokémon per battle.
Unaffected by item negating effects (such as Embargo and Klutz). Item manipulating effects (such as Trick and Fling) cannot give Swampert a Swampertite or remove a Swampertite from Swampert.
Outside of battle
If held by an Swampert, allows it to Mega Evolve into Mega Swampert when it performs a Spectacular Talent during a Pokémon Contest Spectacular while its Trainer has a Key Stone. Swampert will remain a Mega Swampert until the Talent Round ends.