If held by a Latias, the Latiasite allows it to Mega Evolve into Mega Latias 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 Latiasite from Latias.
Outside of battle
If held by a Latias, the Latiasite allows it to Mega Evolve into Mega Latias when it performs a Spectacular Talent during a Pokémon Contest Spectacular if its Trainer has a Key Stone. Latias will remain a Mega Latias until the Talent Round ends.
Data for the Latiasite and the Latiosite exists within the code of Pokémon X and Y, despite the Mega Stones not having been officially introduced until Omega Ruby and Alpha Sapphire, which allows them to be transferred to and used in X and Y.
Latiasite in the Mega Ring System Mega Bracelet Plus Black Metallic set
Latias, Mega Latias & Latiasite metal charm set
