under____(....him....)

Description: *after hundreds of resets,he decided to stop the human once in for all after a pacifist route by killing every human and monster on the surface, during his genocide,he was taken over by gaster,who used his powers to remake the world,turning it into a glitched black and grey desert full of human and monster amalgamates,the fallen and him are the only characters left in the ruined timeline and the fallen one must fight "him" from the surface to the true lab in the underground in order to gain back control of the timeline....or is there a much better way to do this,=)