In the seventh room on the way to the keep.
There stood a man – stabbed in the heart with Akuto the Eel.
Small as a dagger, it sent the lightning it was limitlessly charged with surging through his body, like some kind of an electrode, that he may never feel exhaustion or fatigue, making the man an invulnerable[28] warrior who could fight indefinitely.

[28] 不死身 FUJIMI “deathless in body”

Katanagatari Omnibus 4, Page 285