From Bulbapedia, the community-driven Pokémon encyclopedia.
Lilly of the Vally Island (Japanese: スズラン島 Suzuran Island) is an anime-exclusive location in Sinnoh. The island is covered with mountains and forests. A town is situated on the southern end.
The island is the location of the annual Lily of the Valley Conference and features amenities for competing Trainers just as other league locations do.