The Radiant Citadel was a city-sized locale found in The Ethereal Plane that served as a hub of commerce and culture between over a score of ancient civilizations. After the eventual decline―and in some cases disappearance―of its founder groups, the Citadel also became a repository of multi-planar cultural history and lore from numerous different worlds.
The Citadel as a structure was carved out of the fossilized remains of some unknown entity that formed around the Auroral Diamond. This enormous, indestructible gemstone held profound healing powers and translocation capabilities that allowed for extraplanar travel. The hue of the luminescent crystal fluctuated on irregular time intervals, for reasons that were only speculated upon. The structures and pathways of the Citadel were all carved out the same set of fossilized remains.