WARNING: The following contains spoilers for Episode 48 of That Time I Got Reincarnated as a Slime, now streaming on Crunchyroll.
Rimuru continues to battle his archenemy, Demon Lord Clayman, in the middle of Walpurgis as the other Demon Lords watch. Rimuru, Milim and Veldora have a clear advantage, but Clayman fights on, thinking of his Harlequin allies who depend on him.
Rimuru has recently gained vast political experience, and he needs all of it to negotiate with the Demon Lords and appear legitimate to them. Defeating Clayman will only do so much good; Rimuru must follow up and prove himself a true leader among monsters and demons. That includes his ability to think up creative names.
Rimuru deals the final blow to Clayman with the Beelzebub spell, and with Clayman gone, the council of ten has a free spot. Guy Crimson welcomes Rimuru to the council as its newest member, and no one objects. Among these demons, shows of strength mean everything, and the demons also have a "mind my own business" mentality. None of them see fit to oppose Rimuru's shocking rise to power -- not even Leon Cromwell, one of Rimuru's rivals. Then, two more spots become vacant at the last minute.
Demon Lord Frey, the harpy queen, steps down and requests a position as Demon Lord Milim's subordinate, and Milim reluctantly accepts. Demon Lord Carrion follows suit, and Milim protests until Carrion asks her to take responsibility for her destructive rampage in the Animal Kingdom by accepting him as her newest subordinate and ally. Milim accepts, feeling nervous about all her new responsibilities. Milim has previously been a carefree loner, but she now has a power bloc of her own, becoming a miniature Rimuru Tempest in political terms. Soon, she must prove her capabilities as a leader and diplomat.
The council now has eight members, and Veldora the storm dragon recommends that Rimuru give the council a new name, to which Guy Crimson agrees. Rimuru faces his first challenge as the council's newest member -- he must conceive of a name that pleases all the other Demon Lords or risk disappointing them. He names the council the Octogram, or an eight-star constellation of demons, and the Demon Lords accept the name. Rimuru gets another surprise when Demon Lord Roy Valentine is exposed as a fake: the real one is Luminus Valentine, who was posed as a maid. She resents being outed so suddenly but takes it in stride.
Rimuru has more political power and influence than ever before as an Octogram member, along with a new set of allies and enemies to consider. Milim proves herself a stronger ally than ever, with Frey and Carrion to back her up, and most other Demon Lords appear neutral with Rimuru, including Luminus the vampire.
In contrast, Rimuru and Leon Cromwell may soon come to blows over Rimuru's efforts to block future isekai summoning, as well as Cromwell's role in Shizu's isekai life and her role as Ifrit's host. Rimuru also still has the Western Holy Church to confront, and possibly Hinata Sakaguchi, not to mention the scheming Yuuki Kagurazaka. Rimuru has triumphed over Clayman, but his quest continues into a new era, and new challenges still lie ahead.