Last Breath Joseph, or LBJ for short, is an AU of the original Joseph who was met by Gaster.  Joseph told Gaster about how he was sick and tired about the fallen human getting away with genocide, and Gaster agreed to help him.  Gaster gave Last Breath Joseph the ability to use every attack used by the monsters in The Underground, from Papyrus' bones to Omega Flowey's vines.  This did come at the cost of Last Breath Joseph's face being slightly distorted, but he says it was a small price to pay.