That Time I Got Reincarnated as a Slime season 3

 Our favorite, overpowered, hero Rimuru is back, and ready to advance the alliance ever more.

Now one of the eight Demon lords, he has to learn with the consequences this brings along as well.

But he has his friends and allies, like Diablo.  He has made a plan that will force the king of the country that invaded him to ally with Youm after being removed from the throne.  The kingdom of Falmuth concedes, and agrees to paying repairs to the Jura Forest Alliance, but Hinata the legendary hero is still out to take Rimuru down.  She serves the lady Luminus, a vampiress demon lord also on the council, and who plans to defeat Veldora.

In Tempest, Rimuru most work to solve a lot of day-to-day business issues.  Vesta and Kaijin have developped an anti-magic barrier device to keep monsters from spawning on their lands, as Rimuru starts thinking there might actually be various masterminds operating behind the scenes, considering the timing of the recent events.  Hinata discusses with the other members of the Ten Holy Saints that they should not engage Rimuru, believing he is genuine in his peace with humans plans.

But three Luminaries show up, beings that train the mighty Heroes, sending her on her way to Tempest to take down Veldora, should the rumours of his return be true.  As Rimuru is gearing up his defences, the Rozzo family, a mighty business consortium in the Western Kingdoms and in league with the Eastern Trade Merchants, are on the tail of a demon manipulating Falmuth behind the scenes and plan to interfere.

During Hinata's and her companions trip to meet Rimuru, they come across a well-run and developped land, even serving Ramen.  They find two others of their order, Garde and Renard, engaged with Rimuru's forces, as those have been send unbeknowst to her to take him down.  They meet at last on the battlefield, where Hinata and her saints are defeated by Rimuru.  The Seven Days intervene, trying to assassinate Rimuru as Hinata intervenes and takes the blast, being grieveously wounded.

Diablo engages Edward's army, as the Seven Days clergy appear there as well.  But they are no match for the arch-demon, who with the acknowledgement of Rimuru dispatches the clergy, and makes sure Youm will ascend the throne of Falmuth.  Demon Lord Valentine, aka the godess Luminus, appears to ressurect Hinata and dispatches the clergy to make up for the grievances the Jura Forest Alliance suffered.  

 

In the resulting talks, the Jura Forest and demi-humans in general become recognised by the Western Holy Church by Hinata and Valentine, opening trade relations and involving the Dwarf kingdom as such as well.  To celebrate all the progress, they decide to organise a full fledged festival, Shuna looking for aid from a famous chef from Egrassia. 

The second arc of the season starts with Ramiris moving places, deciding to come and live in Tempest together with Treyni and Beretta.  Rimuru allows her to make a 100 floor underground dungeon, to become an attraction in Tempest and train budding adventurers.  The Empress Elmesia of the independant nation of the Sorcerous Dynasty of Thalion is invited by Rimuru.  She considers this as her sweets supplier from Engrassia has moved to the city as well.  The dungeon is build, helped by the Lycantrophe refugees, in time for a festival they are planning to celebrate the birth of the Tempest realm.

 

Together with Kurobe, the treasure chest loot is made for the dungeon, and Veldora agrees to become it's final boss, allowing a place he can unleash his aura which has been building up to it's limits inside him.  Milim also arrives, wanting to be involved as reports from strange creatures on the edges of the forest.  A trio arrives in town causing some ruckus, before quickly being knocked out by Shion.  They turn out to be the demon lord Daggrulis's sons, send to train, to which Rimuru agrees.  An alliance is also being formed with the Tengu clan, and the Elder confesses she fell in love years ago with an Ogre sword-savant, who turns out to be Hakuro, also the father of her daughter Momiji who is send to Tempest as an envoy.  She wants to win Benimaru's hand to strengthen an alliance, but Lady Albis also has an eye on him and they decide to do their best to win him over.

 

As an audience with all the tribes of the lands is being held, and they are all formally invited to the festival, Masayuki Honso, aka The Lightspeed Hero and also an otherworlder, defeats a circle of slavers and decides to bring them to the Jura forest.  A small intermezzo in the arc is then included, looking back at the first arc's events but from the viewpoint of demon lord Luminus.

 

Meeting with Gazel, the Dwargon king, Rimuru discusses the possibilities opened by the defeat of the Seven Days Clergy.  He go visits Yuuki, and take his young students to the city for the festival, as both the Empress of Thalion and the Lightspeed Hero also arrive at that time.  After a big banquet, the festival officially opens.  In a martial arts tournament, a seat is up to join Rimuru's "Elite 4" of protectors, and Gobta finds himself suddenly in the finale against The Lightspeed Hero.  The latter concedes after seeing the new power Gobta and Ranga have unlocked together, while Rimuru, Gazel and Emrisia theorise about a saboteur working at the festival to financially bring the new realm in disrepute.  

They uncover the culprit, realising he is nothing but a puppet, and deduct after the festival that is must be Yuuki who is orchestrating it all, but the Rozzo family is also involved...

 

And so the second arc, while all centered around the dungeon and the festival being build in Tempest, is actually one big setting the stage for the fourth season.  Alliances are forged, and they realise a big threat is looming behind the horizon, of which Clayman was only a small part of it.  So let's await the new season and see how this pans out...