That island is Otto's dungeon island, made up of three separate pieces of land available for purchase: Dungeon Plateau, Otto's Grotto and Steam Fields. It is home to the Bottomless Dungeon. Each week, a new dragon is available in the Bottomless Dungeon, which you can explore by taking part in the Brawl to earn Dungeon Stepping Stones. With these, you will be able to progress through the Dungeon and open chests, which have a chance of containing the weekly reward dragon as well as numerous other great rewards.
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