Since this story takes place in a crossover world between High School DxD and Jujutsu Kaisen, some changes were necessary to keep the setting balanced. The DxD side, in particular, suffers from heavy power creep, and without adjustments, there would have been no meaningful stakes.
Think of this as an AU. Canon won't help much anyway, this story goes its own way.
You don't need to memorize any of this to enjoy the story, but if you care about world-building, here are the key changes.
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Great Magic Suppression
Roughly six centuries ago, during the Great War, the God of the Bible enacted a final miracle before his death.
He imposed a planetary suppression field over Earth.
To restore balance between humans and supernatural beings, this field causes all magic used on Earth to cost roughly three times more than normal, while also making the learning curve steeper.
DXD magic is brokenly spammable. This change exists so it doesn't bulldoze the entire Jujutsu power system.
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Jujutsu Power System
1. JJK power system is incompatible with DXD power systems. A Jujutsu sorcerer cannot wield Magic (or vice versa). If reincarnated into a species that cannot use Cursed Energy, such as devils, angels, or dragons, they lose their CE and instead gain that species' supernatural energy. Gods are the sole exception.
2. By default, Domain Expansions only work on beings with Cursed Energy. In this AU, that rule is overwritten: Domains affect all supernatural energy systems. The only beings immune are those with absolutely none, like Rein (MC).
3. Only beings with CE or those who possess Heavenly Restrictions can perform Binding Vow.
4. Cursed Spirits can only be truly exorcised through Cursed Energy or Reverse Cursed Technique. Other DXD energy systems can damage them, but this is merely brute force rather than proper exorcism, making it far less efficient.
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Trivia
Ophis does not exist in this world. Why? Because Great Red kicked her out of the Dimensional Gap and threw her into a parallel world.