Crux of Nex

From PathfinderWiki

The Crux of Nex is a demiplane created by the legendary archmage Nex during the centuries-long war with the rival wizard Geb.

Nex was able to use the Crux of Nex to obtain immortality, and shards from the Crux were used to create both the Spire of Nex and the Refuge of Nex.1