From Bulbapedia, the community-driven Pokémon encyclopedia.
Masquerade (Japanese: マスカレード) is one of the Ghost Masters located in the Sealed Fort, located on GR Island in Pokémon Card GB2: Here Comes Team GR! Masquerade can be found in the seventh of the nine statues in the main room. Players who defeat Masquerade will obtain a Present Pack, which can consist of any card that can be found in a booster pack in the game.
Power of Darkness Deck
Power of Darkness Deck (Japanese: あんこくのパワーデッキ) is the deck played by Masquerade. The deck features the promotional Great Rocket's Mewtwo, which was created to promote the game, as well as several types of Dark Pokémon, one of which is also exclusive to this version of the game.