This take when Chara slashed Papyrus, Undyne and Asgore, sans will come up defeated, they come up when a deal to revive the monsters as demons, sans accepts the deal of the idolized standing.

All they can do is just revived those three, and join forces and takes over humanity.

Good ending: He successfully give the soul to Frisk, so he immediately passed away to his normal self, his veins are gone.

Bad ending: His veins were outta control and immediately burns everything into ashes.