A kaishakunin is a man appointed to behead an individual who has performed seppuku, Japanese ritual suicide, at the moment of agony. The role played by the kaishakunin is called kaishaku.
A staged photo from the late Edo period of a seppuku ceremony. The kaishakunin is standing at the rear with his sword raised and prepared to partially sever the head, cutting through the spinal column, of the person performing seppuku.