"Errorfell! Sans!' is the opposite of Error!Sans the opposite of physcotic and demented his Gaster Blasters are bigger then Underfell Sans,s and really dosen,t like Corefell Frisk.
Errorfell! Sans is a puppet master, which using blue strings of his to control someone's soul as well they function as grappling hooks. He also uses Gaster Blasters that move twice as fast as the original ones.