Shiteibushi

Shiteibushi (指定武師, literally "Hand-Picked Martial Art Master") is a term used by to designate eight particular individuals which had once trained under him and were considered to possess remarkable power and skill. In the challenge he posed to the in which he would pass on his own title of  to the one that managed to seize the title of Shiteibushi from every single one of them. Whenever an individual manages to defeat one of the titleholders designated by Seireitou, they take that person's title and it carries over to them. In order for one to complete the challenge, and be chosen as the new Yonkō, they must succeed in taking all eight titles; besides the traditional approach in which one would have to defeat Seireitou himself, which is arguably a far more difficult task.

Behind the Scenes
This was inspired by two different situations that both occurred in the manga and anime series, One Piece. The first one is the challenge posed by Gol D. Rogers, in order to find his treasure and become the next Pirate King. The second source of inspiration, which is arguably darker but fits with Seireitou's libertine ideals, is the "birdcage" created by Donquixote Doflamingo. In terms of the latter, the challenge was either to kill Doflamingo himself or to kill the twelve criminals that he named in order for the game to end and the people to be spared.