Sky Realms Cluster (Endhaven Supplement)
From D&D Wiki
The sky realms are not truly a cluster. The sky arches over all realms equally. Few can ply these realms but the Elemental Lord of Air lifts the city of Astrea into the air and this city visits the far-away realms in the sky. There they harvest alchemical silver, imbue rare incenses, and tame mighty creatures.