How Not to Summon a Demon Lord Ω

Diablo inadvertently rescues the priestess Lumachina Weselia from an assassination attempt. As Lumachina had been praying when Diablo appeared she now believes Diablo is God. Lumachina believes the Head Cardinal of her church wants her dead for investigating his corruption, so she had been looking for Head Paladin Batutta for his assistance.

When the group arrives in Zircon Tower City, a child named Horn offers to be a guide to the dungeon; the description confirms it is Diablo's dungeon. Later, a monstrous sand whale attacks the city and Diablo tries to fight it, but requires the help of Batutta and the city's leader, Fanis Laminitus.

Lumachina and Rem are drugged and abducted by Batutta and his maid, Shiliu. Horn spied on them and warns Diablo and Shera.

The group arrives at Diablo's dungeon and have to go through several rooms filled with mazes, monsters and lava.

Diablo saves an unconscious Horn and sacrifices his shirt and cape to make a fire. Rem, Shera, and Lumachina get through several floors of the dungeon only to run into Gewalt, who attacks them. They are interrupted by a massive black dragon who attacks them all.

Zircon Tower City's defenders are no match for Varakness' army. Varakness attempts to force Fanis to marry him. Diablo takes offense to Varakness saying he is working for a Demon Lord, so his group, including Rose, teleport to the battle. Diablo, Shera, Rem, and Rose then battle the army while Lumachina heals the injured with Horn assisting her.

Lumachina decides to go to the Royal Capital to root out the corruption of the Church, but first the group returns to Faltra City. While Lumachina visits the city's church, Diablo and Rose learn that Edelgard works at a bakery with Klem and Celestine as the usual customers. Klem warns Diablo that she senses something powerful in the Royal Capital.

The group arrives at the Royal Capital, Sevenwall, and are accosted by guards, but Alicia arrives and dismisses the guards, then lets them into an inn she owns. As they eat, Horn says she wants to be stronger. Rose presents a slave collar and says if Horn wears it and makes a Master-Servant contract with Diablo, his power will flow into her, but Rem and Shera warn that if Diablo dies...

Lumachina is separated from the others and sentenced to execution. Diablo orders the others not to escape because Lumachina asked them not to. Horn inadvertently activates the Holy Grail she took from Diablo's dungeon and a tiny fairy-like girl named Babylon appears. She explains she appears to those who offer blood to the Grail.

Vishos merges with Europa and boasts that he is a God, then starts smashing the church while trying to kill Lumachina. Diablo learns he is protected by a force field, so he asks Lumachina to cast Dispel to destroy the force field, but must protect her as it takes a long time to cast.