This is underswap Sans but after killing his brother he gained dust, he fought you and almost lost BUT he decided to absorb the human souls and is now Giant.
He looks like a giant skeleton dragon with a gaster blaster as his face.
His abilities are Gaster blasters, Bones, Soul powers, Reality manipulation, Computer control and much more
His final goal is to take your soul and break the barrier.
His isn't evil he just wants to escape the underground to save all monsterkind.