Nekronopolis

During his brief solo campaign after his escape from the ruins of Mt. Fate, Enerjak used his powers to create a city called Nekronopolis upon the site where Echidnaopolis originally stood. It was meant to be the capital of his new empire, but instead was the battleground where he faced Knuckles in a brutal hand-to-hand contest. When Enerjak was launched into space aboard his citadel, courtesy of Locke and the Chaotix, the rest of the city disappeared. The citadel itself has either similarly vanished or remains in orbit above Mobius, due to Enerjak having been brought back to Mobius by Mammoth Mogul.