A recent datamine from Pokémon Go player Chrales showed that “shadow Pokémon” are coming to the mobile game.

What this means isn’t entirely clear, as all we have to work off of is some dialogue that Chrales dug up. According to the datamines, posted to Twitter, Team Rocket grunts will appear at Pokéstops and upon defeating them through an unknown method, they abandon the Pokemon they battle against you with. The Pokémon in question has an “ominous aura and is somewhat different from regular Pokémon.” The datamines also mention purifying the Pokémon, but its unknown how players will do this.

This entire process sounds similar to Pokémon Coliseum, a GameCube game from 2004. The game involved rescuing and catching other trainers’ Pokémon that have turned into Shadow Pokémon. Shadow Pokémon were turned this way to become stronger, but in the process are unable to bond with their trainers. In Coliseum, Pokémon were purified by using them in battle or setting them up in a purification chamber.

Team Rocket were originally teased during Pokémon Go Fest in Chicago, photobombing people’s AR photos. They also made an appearance in the form of a Team Rocket hot air ballon at Dortmund, Germany’s Go Fest. A couple of grunts were also spotted sneaking around in the art for the game’s anniversary earlier this month.

We don’t know when Shadow Pokémon will launch in Pokémon Go, but we can expect it to be sooner than later, as most of the dialogue is already in the app data.