Void Caster is a minor character who appears in the episode "Escape from the Citadel."
The Void Caster appears as a large, blue-skinned humanoid creature with a stocky capsule-shaped body, beefy arms and legs, and a domed, neck-less head covered with short spikes. It has no visible facial features, but it is capable of speaking in a strange alien language. It wears a black tattered toga.
The Void Caster wields a magic wand with which it can open portals to various places, which it uses for attacking, defending, and escaping. During "Escape from the Citadel," the Void Caster is seen opening several portals that appear to lead to deep space, as well as one portal that opens up to Ooo, out of which a train driven by Starchie appears and hits one of the Citadel Guardians. He later helps Martin and several other cosmic criminals escape the citadel.
Not much is known about the Void Caster other than his powers. He was imprisoned in the Crystal Citadel for an unknown Cosmic Crime, but escaped once the Lich arrived and took over its mind along with the minds of the other prisoners in order to destroy the Citadel. Once Finn defeated the Lich, all the criminals were free from the Lich's power, and the Void Caster used its portal-making magic to help Martin and the other criminals escape.