Heaven is the final area of Monster Girl Quest, home to the Angels as well as where Goddess Ilias resides and watches over the world from. This is where the final battle against Ilias and her commanders is fought.
When Luka enters Heaven (through Heaven's Gate), Seraphim Eden confronts him. Searing with anger towards the son of Lucifina, she attempts to "imprison" him within herself so that his "heretical soul" would never reincarnate, but she is ultimately defeated by Luka and Alice.
Proceeding further into Heaven, Luka and Alice encounter Promestein. Promestein reveals that Ilias was absorbed by Black Alice, who had injected herself with the White Rabbit drug. The "fusion of the Dark God and Ilias creating the ultimate existence!". However, unbeknownst to Black Alice, Promestein created a second White Rabbit optimized for herself. Promestein pierces her neck with the syringe and injects the White Rabbit into herself, transforming her. However, Promestein states that Black Alice is still stronger than her. But if she were to absorb Luka, the son of one of the first angels created by Ilias, Lucifina, the holy-dark fusion would ascend her to Black Alice's height. And with that power, she would defeat Black Alice and rule herself. However, this would never come to pass, as Luka and Alice strike down Promestein once and for all. And while melting away, she declares all of humanity as her successor, believing that their thirst for knowledge will eventually surpass Promestein's work and research.
Proceeding further, Luka and Alice encounter Black Alice. And after a fierce fight, Black Alice pushes her power "past the critical point", causing her body to endlessly expand at an explosive rate. And if not defeated, she threatens to consume the world. However, Luka and Alice strike down the former Monster Lord once and for all, Alice declaring "You hold nothing now. Now vanish into the darkness of history."
But while Black Alice's body disintegrates, Goddess Ilias rises from the mass of flesh, revealing that Promestein's and Black Alice's betrayal was all part of her master plan to become the ultimate deity herself. As Ilias could not directly integrate with dark energy, she required Black Alice to devour her, and then needed Luka to defeat her, freeing Ilias. And with her newfound power, she would be able to wipe out all Monsters, including the unsealed Dark God Alipheese and the Six Ancestors.
And so, with the weight of the world on their shoulders, Luka and Alice rush into the final battle to decide the fate of the world. And with the help of the Four Heavenly Knights and Micaela, Luka and Alice defeat the tyrannical Goddess, finishing her with Quadruple Giga, infused with holy and dark energy from Luka and Alice.
And with Ilias's death, Heaven disappears.