Demon Island, formerly known as the Sunken Island is Anti-Mobius's version of Mobius Prime's Angel Island. As its name suggests, it is an island sunken under the sea with an air bubble to provide its occupants with air. The Demon Island's history is never explained. Like its Mobius Prime counterpart, it is also home to the Echidna people, with O'Nux serving as the guardian of six Anarchy Beryl.
The Sunken Island was conquered by Scourge the Hedgehog during his takeover of the planet, after defeating O'Nux. It is presumed it was liberated after Scourge's defeat.