Githyanki

Githyanki are a medium sized humanoid race who originate from the Astral Plane.

Known Githyanki
Vox Machina found a githyanki skull in General Krieg's home. They learned that a githyanki was trapped in a jewel embedded in a skull. It promised to grant a wish to them if they released him.

Matt revealed after the campaign concluded that the spirit trapped in the skull was that of brutal githyanki pirate captain imprisoned generations ago. Had anyone agreed to free him, their souls would have exchanged places, with the captain taking over the body of his liberator whose spirit would have been then trapped in the skull. The githzerai who had imprisoned the pirate would then have begun hunting him down.