The story starts out with Yusuke questioning his own purpose in life. Yusuke had previously killed Sensui indirectly by his demon's ancestor, Raizen which is one of the 3 kings in Demon's world, but not himself, and has no idea what laid in his future. Spirit world see him as danguous as other demon and fired him as a spirit detecter, just the very next moment Yusuke, Kurama, Hiei got invitation from the Demon's world 3 king separately.
In Demon's World there are three extremly powerful rulers Raizen, Yomi and Mukuro. They each have an equal share of land and they rule over different variety of demons. Raizen is the strongest of the three, and Yomi is the newest to obtain power. The 3 kings cannot really start war to take over's eachother because if 2 of them fight, the remining one will attack the one who won but already weakened. So the 3 kings situation was hold for more then 1000 years. The whole conflict began over the argument of eating humans. Raizen said that they should stop eating them, while the other two disagreed. Raizen getting weaker since he hasn't eat for over 500 years, and he is going to die soon and the balance of the 3 kings demon world will be changed.
Yusuke going to Raizen's country, Kurama going to Yomi and Hiei going to Mukuro. They were friends and now they are in a diffcult situation. This storylines has a lot of political factors but however, Mr. Togashi wants to finish the story shortly at the end. The story was still really attrative and the characters Mr. Togashi created are always so charming. And the end after Raizen dies; Yusuke suggested organizing to have a Tournament to choose the new king. Of course it's not as easy as he think as this thought is rather innocent or naive in demon's eyes, however with his friend Kurama playing political card and the cool of one of the remaining king Mukuro, the tournament started.