Crystal sphere

Crystal spheres stand in the Prime as nearly-impenetrable shells many thousands of miles wide that surround the various regions of Wildspace and hold the phlogiston back. Formed from an unknown material pure black in hue, nothing beyond the obvious is known about the substance that makes up these spheres, as no mortal force has been yet found that can so much as scratch them; the only means by which a crystal sphere can be left are spells of transportation or natural openings found across the sphere, and not even the force of a god seems capable of affecting this substance. In fact, the crystal spheres represent the outermost boundaries of the influence of a world's deities on the Prime, with deities having no influence even just beyond their edge.

On most worlds, the inner surface of the world's crystal sphere is the source for the majority of the starscape that fills their night skies. The exact nature of the stars varies from space to space, ranging from glowing gems embedded into the surface to clear portholes onto the phlogiston to the light of alien civilizations spanning the surface of the sphere.

Reference

• Adventures in Space - The Concordance of Arcane Space, pg.9