Giant horned skulls used by the character Sans, the final boss of Undertale on a genocide run (Killing as many monsters as possible).

Often depicted as floating, these skulls emit laser-like blasts from their mouths that damage the player.

"Gaster Blaster" comes from the name of their sprite in the game files. The name seems to imply these skulls have connection to the secret character W.D. Gaster.

Related:

• Sans_(Undertale)
• Gaster
• Undertale