The Aurum Island is one of the fortresses the Aurum created as to control their fleet on their first attack on Earth. It is found in Chapter 15, Mysterious Invaders. The Aurum Core is located here, and Pit eventually destroys it, preventing the Aurum from extracting more Earth materials than they already have done.
The Aurum Island is one of a few that are witnessed during the Air Battle at the beginning of the chapter. However, it is on this unnamed specific island that the Aurum Core is found on. When first arriving on the surface, Pit himself asks Palutena if it was the future that lay before his eyes. It is on this island as well that many new concepts and designs of the Aurum are first revealed. As stated in the Idol Description, the islands created have a shabby patchwork look due to the different materials harvested from different planets. Even so, they still manage to keep up a technical look. Aurum Islands may look pretty on the top, but the bottom of the Island is a collection of jagged rock shards and enormous, irregular stone towers. The whole island is held up, floating, thanks to a mysterious Aurum anti-gravity field that has yet to be explained.
The Aurum travel with vast floating islands like this one. Similar to how bees gather pollen, the Aurum harvest material from worlds to create even more islands. This gives each island a strange, patchwork look.