Rimuru goes undercover in a kingdom tearing itself apart, and the OVA actually knows how to pace a three-episode mystery without letting the plot sag under its own weight. You get the character action beats you came for, Soei and Benimaru earning their screen time, while finally getting real answers about Luminous that the main series kept locked away. It's tighter than Scarlet Bond, which means it feels less like filler and more like the story remembered why you started watching in the first place.

Rimuru's teaching duties in Engrassia are winding down, now that he has saved the children in his care, when Yuuki, the Free Union's Grandmaster, recruits him for a new mission. The Kingdom of Coleus lies between Engrassia and the Holy Empire of Lubelius, and a struggle over who will take its throne is under way. Using the name Satoru, Rimuru slips into Coleus to learn what is really behind the succession fight. What he uncovers is far more tangled: demons and vampires are pulling strings of their own, and the kingdom is caught in a web of their plots. Set between the first and second seasons, this three-episode OVA mixes action, adventure, comedy and fantasy.