Dark Sun: The Scorched World of Athas
The Zwuun appears as a cloud of steam, dozens of translucent human and demihuman faces drifting randomly within. It has no physical substance.
The Veiled Alliance of Nibenay employs (or tries to employ) this ancient spiritual force. This being resides in the hot springs outside the city. Alliance members never feel certain of their success when enlisting this fickle creature.