The ending of 100% Pacifist Deltarune will essentially be the ending of Gurren Lagann where literally everyone you've met, Lightners and Darkners alike, combine with the Thrash Machine using the power of your soul in order to fight a Titan controlled by and/or containing the empty vessel you created at the start of the game, ending with either Kris or Susie ramming you into said vessel and ending the Roaring in the process, and giving you the means to live your own life without needing Kris as a host.
Oh, and Ralsei will probably sacrifice himself during the ordeal at some point too.