If you never caught deoxys when he first showed up in the end of the game, in Omega Ruby and Alpha Sapphire. All you have to do is catch all the legendary Pokémon in that game except obviously the ones that don't go in that game. The legendaries that do appear in the game. You then go back to Sky Pillar, and all the way to the top. There you find this black triangular black floating thing. You go up to it and click A and then it will explain about a Pokémon. That's how you get deoxys!