Post-war arrangements following the battle with the Orcs have begun. At the meeting, Rimuru reveals that he made a promise with the Orc Disaster, Geld, to take all of the Orcs' sins upon himself. When the Ogres and Lizardmen hear this...

The Jura Tempest Federation, with Rimuru as its leader, has formed an alliance with the Armed Nation of Dwargon. Just as Rimuru thinks there are only peaceful days ahead, he senses an enormous mass of magical power heading straight for his location at an incredible speed. When the smoke from its collision with the ground clears, standing in its place is a humanoid figure. She is the Demon Mord Milim, the one and only Dragonoid and the one bearing the nickname "Destroyer." Ranga, Benimaru, and the others who face her are quickly overwhelmed by her power.

The Demon Lord Carrion has sent one of his Three Beastketeers, Phobio, on a raid of Rimuru's city. However, Phobio is quickly defeated when Milim meets his attack with her own. Rimuru and his subordinates then ask Phobio why he has come to their town.

A merchant from the east brings Hinata Sakaguchi a new book. In it she finds surprising information about the Jura Tempest Federation, a nation of monsters. As a former student of Shizu's, what goes through Hinata's mind upon learning about the existence of Rimuru? This special episode gives some hints as to what's coming in the new arc, as well as a look back at Rimuru's exploits thus far.

Despite the initial conflict, Eurazania and Tempest have deemed each other to be worthy of establishing friendly relations, and diplomacy between them officially begins. Tempest agrees to share its industrial techniques in return for Eurazania's agricultural techniques, and Rimuru decides to use the apple brandy produced as a result of this exchange to seek friendly relations with the Armed Nation of Dwargon next. He's accompanied by Shuna and Shion along with the former residents of Dwargon, Kaijin and the three Dwarf brothers, who are welcomed back to their home nation as state guests.

Following Clayman's orders, Myulan ignores the words of Youm and Grucius, both of whom have fallen in love with her, and activates the Anti-Magic Area spell. At the same time, Reyheim also activates Prison Field. The double barrier prevents Shion and the other fighters of Tempest from fighting as well as usual, and they end up at a disadvantage in their fights with three Otherworlders. Benimaru tries to contact Rimuru, who still hasn't returned, but he can't even get in touch with him due to the barriers.

Rimuru announces his plan to back Youm as the new king of Falmuth. After hearing this, Gazel tests Youm to see if he has what it takes to be a ruler, and Youm answers with full sincerity. Erald also takes a liking to Rimuru after hearing him say he just wants to create a world where everyone can prosper and smile. Thus Rimuru and his allies look ahead to the battle against their new enemy, Clayman.

Benimaru leads his army to Eurazania, and Youm heads to Falmuth to take it over along with his buddies and Diablo. Rimuru succeeds in outwitting Clayman by transporting all of Eurazania's refugees to Tempest. Since Clayman has been striving to evolve into an Awakened Demon Lord for a long time, his resentment toward Rimuru grows. Meanwhile, Demon Lord Leon, Veldora's sister Velzard, and Guy, the oldest of the Demon Lords, become increasingly impressed with Rimuru.

Gabiru narrowly wins his battle against Hermes. Watching their outcome is Middray of the Dragon Faithful, whose skill was enough to overwhelm even Suphia. Though he looks like a human, he is actually a Dragonewt like Gabiru. Meanwhile, Yamza uses his artifact Doppelganger to create a double of himself, but Albis still defeats him soundly. He tries to surrender, but Clayman takes control of him and forces him to transform into Charybdis.

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.