Tensei shitara Slime Datta Ken

Synopsis

A dying woman summons a demon to avenge her and her partner and the demon agrees to her request, deciding it would be an amusing distraction. Shizu's memory: Shizu is summoned to the Kingdom of Filtwood with other adventurers to stop a demon from taking its remains under the castle and reviving. Upon hearing the Silver Wings, a pair of famous adventurers, were killed by the nameless demon, some try to withdraw. The minister refuses to let them leave, revealing some are possessed by demons, which causes paranoia among the group. One of the crowd, the demon from before, starts laughing, claiming the knights summoned the lesser demons but does not explain. Hired to kill a demon, he reveals he intends to kill all the other adventurers present to find the responsible party. Shizu refuses to let that happen and duels with the being, who calls himself "Kuro." One of his blows is reflected by Shizu's mask and takes off his arm. He decides to withdraw, saying the fight was amusing because Shizu was able to hurt him. Afterwards, Shizu recalls how Leon told her the most powerful demons were the progenitors, known by colors like Rouge (red) and Noir (black). One of the knights summons her to king and takes her beneath the castle. It is revealed the king and minister were puppets of the same demon she was hired to stop, who has taken on the persona of Orthos, the hero who previously defeated him, thus gaining the name Orthos and becoming an archdemon. They intend to sacrifice the adventurers and Shizu, who is still worn out from her fight, to regain his full strength. Orthos is able to defeat her but Kuro timely appears and kills the king and minister. He reveals he was summoned by the dying Silver Wings to avenge them and kept his promises. He scolds Orthos for being overconfident despite being a dependent of Rouge and destroys his soul. Officially, the king and minister were killed by Kuro, who Shizu then defeated. As long as they keep their stories straight, Kuro says he has no reason to target Shizu before leaving. In the post-credits, Kuro observes Rimuru through a scrying glass and declares that next time Rimuru will lead him "to the truth of this world."

(Source: Wikipedia)