Astral Plane

The Astral Sea is a dimension outside of the Material Plane.

Lady Allura told Vox Machina during a conversation on how to dispose of the Horn of Orcus they had acquired that, when a Bag of Holding is destroyed, its contents jettison into the Astral Sea.

Creatures
It is unknown what beings reside within the Astral Sea besides the one Lady Allura mentioned.
 * Githyanki