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The Void is the space between universes, composed of everything and nothing at once. It is filled with eldritch beings, the Gaster Blasters some of them, and cannot be lived within, nor entirely comprehended, by living creatures from physical worlds; to attempt to do so is suicide, as the Void itself is toxic and mind-altering. It can be traveled through, as evidenced by Sans' teleportation abilities, and is home to Gaster, So Sorry, Glyde, and the Grey Heralds, beings no longer of the Underground, nor truly living, so as to suffer its effects (at least mostly).

It is the poisonous fumes and mists of the Void that brought about the illness called the Hunger, allowed to spill into the world through the Glitch that Flowey created when he stole the human souls.