Rimuru succeeds in collecting enough souls to carry out his plan to evolve, and the Harvest Festival begins. But he's suddenly overcome by intense sleepiness at the same time, so he summons a demon to stop Razen, the one target who escaped alive. Then, upon returning to Tempest after successfully becoming a Demon Lord, Rimuru sets about attempting to revive Shion and the rest of the fallen. Through Raphael, the evolved form of Great Sage, the Secret Arts of Spirit Resurrection and Revival of the Dead are activated...

Shion and the rest of the fallen are revived, Diablo has joined the team... It seems as if all is going smoothly, but Rimuru then receives a report that Milim has declared war and Eurazania has been destroyed, and there's still the matter of the Western Holy Church's hostility toward monsters, even with Falmuth out of the picture. But at the same time, Raphael informs him that the Appraisal of Unlimited Imprisonment has been completed. At last, two years after Rimuru's reincarnation into this world, he can finally fulfill the first promise he made here: to free Veldora.

Recap of events from season two, episodes one through twelve.

A feast is thrown to celebrate the revival of Tempest's fallen and the return of Veldora. After enjoying a brief, peaceful respite, Rimuru tells everyone his future plans: he's going to get the word out to the whole world that he's a Demon Lord and declare war against Clayman. Then King Gazel of the Armed Nation of Dwargon, Fuze of the Kingdom of Blumund, and Eren's father, Archduke Elalude of the Sorcerous Dynasty of Thalion, all arrive in Tempest, and an important meeting to discuss the nation's future begins.

As prominent figures from several nations assemble in Tempest for a meeting, Laplace and his companions formulate a plan centered around Clayman. Then someone familiar joins the discussion...

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.

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?

To put a stop to Clayman's plans, Rimuru uses transport magic on a large scale to both move Tempest's army to Eurazania and evacuate the people of Eurazania to Tempest. Rimuru also decides to crash Walpurgis, the Demon Lords' banquet, in order to settle the score with Clayman personally. Since this will be a return match for Shion, he decides to take her and Ranga along. New military forces are organized in Tempest in preparation for the battle against Clayman's army. At last, the eve of battle is upon them...

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.

Rimuru continues with preparations for Walpurgis, including making Treyni evolve into a Dryas Doll Dryad. Meanwhile, Benimaru and his forces have arrived in Eurazania via transport magic, and they promptly run Clayman's army into a trap set by Geld. The Three Beastketeers, determined to rescue Carrion, set out to subdue the enemy army's leader. Albis fights one-on-one with Yamza of the Five Fingers, while Phobio takes on Footman and Tear.

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.