Clusterspace

Clusterspace, also known as the Astromundi Cluster, was a remote and extremely ancient crystal sphere, regarded by some sages as the place where mind flayers originated.

Description
Unlike other crystal spheres, the crystal shell of Clusterspace was easily traversable from the phlogiston, but it was immune to any attempt at breaching it from the inside. The only devices that could freely exit the Astromundi Cluster were living spelljammers, such as those of elven design.