Isekai Maou to Shoukan Shoujo no Dorei Majutsu Ω

Synopsis

Zircon Tower City's defenders are no match for Varakness' army except for Fanis. Varakness and his wives: a mermaid, a harpy, and a dragon-like banshee appear and attempt to force Fanis to marry him. Fanis shoots him through the heart, but the mermaid heals him. The harpy becomes jealous and attempts to kill Fanis. Diablo takes offense to Varakness saying he is working for a Demon Lord, so his group, including Rose, teleport back in time to save Fanis. Diablo, Shera, Rem, and Rose battle the army while Lumachina heals the injured with Horn assisting her. Diablo is enraged when he sees Varakness flaunting his wives as it reminds him of how he was unable to get a girlfriend in real life. He engages them and quickly kills the wives. Enraged, Varakness attacks but is no match. Diablo kills him while declaring himself the true Demon Lord. With the battle won, Fanis holds a feast in celebration. On the way to the feast, Horn is intercepted by bullies who beat her up and ridicule her and her friends, saying they don't believe Diablo actually saved the city. Diablo rescues her and beats up the bullies. As he carries the injured Horn, she laments that she is so weak, but he says that is okay and she can stay with his group.

(Source: Wikipedia)