If held by a Mawile, the Mawilite allows it to Mega Evolve into Mega Mawile if its Trainer has a Key Stone. A Trainer can only Mega Evolve one Pokémon per battle (or only have one Mega-Evolved Pokémon at any given momentZA).
As a Mega Stone, it is unaffected by item-negating effects (such as Embargo and Klutz). Item-manipulating effects (such as Trick and Fling) cannot give or remove a Mawilite from Mawile.
Outside of battle
If held by a Mawile, the Mawilite allows it to Mega Evolve into Mega Mawile when it performs a Spectacular Talent during a Pokémon Contest Spectacular if its Trainer has a Key Stone. Mawile will remain a Mega Mawile until the Talent Round ends.
Erases Pokémon with three lines from upper right to lower left. Formerly, the description was "Erases diagonal Pokémon from upper-right to lower-left corner." prior to Version 1.2.0 (3DS)/unknown (Mobile).