The Immortal host.

The Kessen-Chu, are the blood worms that live inside Manji.
re-attaching severed limbs, closing opened wounds by replacing lost tissue with their bodies, etc. This is only possible during infestation of course.
They are ,in fact, the reason he is Immortal.

_{Did it during a sermon at church, while 4 year olds were looking over my shoulders. Being a responsible and considerate adult, I pinched their noses while scolding them about the dangers of pornography. They got bored, ignored me, and turned around.}

problem solved 8D

_{(c) Blade of the Immortal}