From Bulbapedia, the community-driven Pokémon encyclopedia.
Cerise Island is a location in the Pokémon Special manga. It was located somewhere in the oceans near Kanto. The island is unmapped and almost unknown.
It was the headquarters of the Elite Four, whose plans were to decimate the entire human race save for themselves and their allied trainers. They felt that humans were destroying Pokémon and only by killing all humans could the Pokémon be saved.
Cerise Island appears in the Pokémon Special manga, volume 4-7, where Yellow battles against Lance in order to save the human race. [[Lugia (Pokémon)|]] also makes an appearance above Cerise Island.