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...

The battle for revenge has begun. Benimaru and his trusted companions carry out their respective missions, while Hakurou and his party face Shogo and Kyoya once again. And Rimuru's fury rains down upon the army from Falmuth.

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.

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.

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.

The majin Gelmud makes his appearance at last. His true goal was to evolve the Orc Lord into the Orc Disaster, a Demon Lord, and have him reign over the Forest of Jura. Upon learning this, the Orc Lord takes action...

At the same time as the attack on Tempest, Rimuru is also under attack. With one look, Rimuru realizes that his assailant is one of Shizu's former students, an Otherworlder, and the woman then introduces herself as the strongest of the Holy Knights, Hinata Sakaguchi. Rimuru is backed into a corner when her Holy Field seals most of his magic. At the same time, Shion and Hakurou, whose magic has been sealed by a similar barrier, are backed into a corner by Shogo and Kyoya.

Rimuru faced grave danger in his battle with Hinata, but he managed to escape in the nick of time by using a doppelganger to take the damage while he rushed back to Tempest. Upon arrival, he found the city in a tragic state, engulfed by blood and smoke. Benimaru tries to capture Myulan, the top suspect behind this disaster, but Youm and Grucius stand in his way. Swallowing his rage and grief, Rimuru realizes who their true enemy is and calmly questions Myulan instead of punishing her.

Rimuru learns about Clayman and the schemes of the Kingdom of Falmuth, but no matter how hard he wishes for it, his fallen comrades and their peaceful lifestyle will never return. He's just about to give up when Eren tells him of a fairy tale that might offer a glimmer of hope. The probability of success is about 3%... but if Rimuru can manage to become a Demon Lord by taking in the souls of 10,000 humans as nourishment, he just might have a chance at bringing his comrades back to life. As Falmuth's army of 20,000 men marches toward Tempest, Rimuru takes hold of that hope and decides to become a Demon Lord.

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.

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.

Rimuru and his Goblin riders set out for the marshlands to finalize their alliance with the Lizardmen. On the way, they save a Lizardman who'd been attacked by the Orcs. The victim turns out to be Gabiru's younger sister, who reveals that Gabiru led an insurrection and left to fight the Orc Lord without waiting for the alliance. She also pleads with Rimuru to save Gabiru and the rest of her clan. Meanwhile, Gabiru faces the massive Orc army and is shocked when he witnesses the power of their unique skill, Starved.

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.

The Crusaders have reached Tempest, and the battle is on. Hinata lays everything she learned from her mentor, Shizu, on the line in a life-or-death showdown against Rimuru.

When Shizu was still with Demon Lord Leon, it was a Hero who rescued her. That Hero was Shizu's inspiration to help as many people as she could for many years. She even worked as a teacher for children who'd come from other worlds. And now, as her time comes to an end, Shizu asks Rimuru for one last favor...

This story takes place before Rimuru's reincarnation. Shizu, also known as the Conqueror of Flames, responds to an urgent request calling her to the Kingdom of Filtwood. When she reaches the castle, many adventurers are already there. What request could have demanded such a huge gathering? It turns out a demon has been revived, and the adventurers have been called to subjugate it. It is said that more than ten of the most skilled adventurers have already been killed by this demon. But there is more to this request than meets the eye...

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.

Diablo descends before the enemy, King Edward of Falmuth. Also present are Saare and Glenda, two of the three Battlesages who are regarded as the most brilliant among the Master Rooks.

The Demon Lord Leon Cromwell summoned Shizu out of Tokyo as it burned during in the air raids, then forced the high-ranked spirit Ifrit to possess her. After taking over Shizu's body, Ifrit begins to rampage.

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.

Just after the end of Hinata and Rimuru's duel, Hinata's sword moves on its own and targets Rimuru. Hinata jumps in its way to protect him and is gravely injured. Then the masterminds behind the whole conflict show up...

Rimuru's village has gained Benimaru and the other five Ogres as its newest allies. Soon after, the Lizardman Gabiru visits the village, having heard rumors about it. Gabiru has been visiting Goblin villages throughout the Great Forest of Jura to enlist their aid in fighting off the Orc army. When he faces Ranga with an attitude of provocation, Ranga tells him that if he can defeat Gobta, they'll hear what he has to say...

Rimuru and the Goblins' village is developing nicely. The Dwarves are building all kinds of houses and furniture, and the local Goblin Lords have appointed Rigurd as the Goblin King. As Rimuru watches everything come together, he decides to return to the cave and test the skills he received from Shizu. While there, he receives a request for help from Ranga through Thought Communication, and he rushes back to find his friends facing six Ogres. One of them orders Rimuru to "reveal his true identity." What is going on in the Great Forest of Jura?

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.