the possessor's soul literally merged into ash's, so you'll just rip out his own soul with it, or make it unstable and turn him insane (which is what will happen)
Hex is mentally corrupted by his COUNTLESS GENOCIDES, along with having a small bit of Ichor in his blood.
Flowey, well, you know Flowey.
Dandy and Pebble both have been overcome with ichor, causing them to become twisteds.