Deoxys is a Mythical Pokmon that can be obtained in Pokmon Emerald by completing the Birth Island event. To start the event, the player must have a Jirachi in their party and speak to Professor Birch in his lab in Littleroot Town. Professor Birch will then give the player the Old Sea Map, which can be used to sail to Birth Island.
Once on Birth Island, the player must navigate through a maze and solve puzzles in order to reach the summit of the island. At the summit, the player will battle Deoxys. Deoxys is a powerful Pokmon, so it is important to be prepared for a tough battle.