Sunlight Island is an island in the Hundred Isles. It was being used as a Conqueror base as of Against the Tide, and was where the White Conch was located.
In Against the Tide, Abeke, Conor, Kalani, and their spirit animals go to Sunlight Island to retrieve the White Conch. They needed the White Conch to call the King Ray and find Mulop. Kalani says that some people in her village have started calling Sunlight Island "Monster Island", due to the Conquerors who have taken it over and used it as a base.