A new force has suddenly risen to power in the Great Forest of Jura, and its leader, Clayman, proposes that the Demon Lords hold Walpurgis with the goal of sanctioning the slime who deceived him. Rimuru also begins to prepare for a battle with Clayman, but observing the enemy army's movements reveals that Clayman's target is not Tempest, but Eurazania. Clayman's army will arrive in Eurazania within two days. Can Rimuru's forces get there in time?

With all the preparations complete, Rimuru heads out to attend Walpurgis with Shion and Ranga, while Ramiris takes Beretta and Treyni. In the meantime, Shuna, Hakuro, and Soei find both their vision and their magic detection abilities hindered by a thick fog as they draw closer to Clayman's castle. From within the fog appears the Index Finger of Clayman's Five Fingers, Adalman, also known as the White King. Hakuro and Soei hold the enemies around them at bay while Shuna faces Adalman one-on-one.

Clayman tries to convince the other Demon Lords that Rimuru should be purged, but Guy tells him that if he is truly a Demon Lord, he should defeat Rimuru himself. What should be a one-on-one battle between Rimuru and Clayman is joined by Milim, who has been brainwashed by Clayman, so Rimuru takes on Milim while Shion faces Clayman. But Rimuru is at a disadvantage against Milim and finds himself pushed back... until Veldora bursts onto the scene and takes Milim's attack in Rimuru's place.

Clayman and his pawns stand no chance against Rimuru and his allies. At a loss, Clayman orders Milim to use Stampede to kill everyone present, but Milim reveals he was never controlling her from the start. It was all an act to expose Clayman's plans and identify the mastermind behind it all. Even Carrion is still alive and well. On the brink of death, Clayman uses the souls of everyone he has killed as energy for his awakening. Thus the final battle to settle the score between Rimuru and Clayman begins.

Rimuru backs Clayman into a corner trying to get him to reveal who he's working for, but even on death's door, Clayman's lips are sealed. Left with no other choice, Rimuru finishes Clayman off. With Clayman dead and Carrion and Frey announcing that they're stepping down as Demon Lords, the total number of Demon Lords including Rimuru is reduced from ten to eight. Guy coerces Rimuru into coming up with a new name for their group, and Rimuru's solution is "Octagram," representing each of the Demon Lords as an eight-pointed star. Thus a slime became one of the eight Demon Lords, marking the start of a new age.