The Kogi Kaishakunin is a fictional position appearing in the manga Lone Wolf and Cub. In the story, it was the Japanese shogun's official executioner.
A position of great power, the Kogi Kaishakunin wore robes emblazoned with the shogun's personal crest and was the personification of the shogun as he assisted daimyo during their seppuku. Seppuku was a method of ritualized suicide the Bushi and Samurai used when confronted with shame or defeat, or when ordered to do so by their master. Seppuku offered an honorable, albeit excruciatingly painful, death.
Samurai committing seppuku often had a second, or kaishaku, who would assist their death by striking a fatal sword blow, often severing the head, at the moment of greatest pain, thus expediting the death of the samurai. However, because loyal retainers had sworn never to raise their swords against their master, a daimyo committing seppuku could not have a loyal retainer help him. Thus the Kogi Kaishakunin, being the personification of the shogun, was the only one capable of assisting a daimyo during his seppuku.
In the story of Lone Wolf and Cub, Yagyū Retsudō, the head of the Yagyu clan, conspired to take control of this post as part of his overall plan of taking control of the government. The Yagyu had already subverted the information gathering and secret police equivalents of the shogun, and with the post of executioner under their control, they could protect sympathetic daimyo by faking their deaths and then incorporate them into their shadow government while fabricating evidence and killing those opposed.