Pokémon Tourmaline is a ROMhack of and loose sequel to Pokémon Emerald. This game is intended to take place in an alternate timeline where the setting and events have diverged a bit from the official games. Most notably, some mechanics from later gens are absent while others have been reworked, and a number of new mechanics have been introduced.
It's been several years since the battle of Groudon and Kyogre wrought havoc on the Hoenn region. Humanity's understanding of Pokémon has deepened as more discoveries are made about the origins of Pokémon. A recent surge of discoveries in the nearby Issho region has drawn researchers from all over the world, including May, now an assistant Pokémon professor. After receiving a Pokémon from her, the player sets out to investigate the reports coming from all over the Issho region of new species of Pokémon, new types of Pokémon, and even more strange phenomena. Meanwhile, a new threat is on the rise in Issho, a mysterious crime syndicate who use exceptionally powerful Pokémon.