If held by a Slowbro (or is present in the player's Bag)PE, allows it to Mega Evolve into Mega Slowbro 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 Slowbro a Slowbronite or remove a Slowbronite from Slowbro.
Outside of battle
If held by a Slowbro, allows it to Mega Evolve into Mega Slowbro when it performs a Spectacular Talent during a Pokémon Contest Spectacular while its Trainer has a Key Stone. Slowbro will remain a Mega Slowbro until the Talent Round ends.
Adds one more Mega Slowbro above the match. Actually converts the tiles directly above all other Mega Slowbro into a Mega Slowbro. This effect can replace any disruption in the tile above and counts as erasing the content of the tile for damage, but does not affect black-cloud disruptions. Does nothing if the above tile is empty.