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Frank had decided that for Hades to properly help in the creation of the new Transmigration Cycle, that he needed to have his own body to control Divine Energy.
If he were to simply guide Frank, Frank was not perfect, meaning that he could easily commit a mistake and ending up ruining everything.
Due to this, Frank considered that this could be the right moment to give Hades the power to have his own pseudo-divine body.
He had already given him a mortal vessel in the powerful skeleton he created through Phantasmal Boneyard.
Now, Frank simply had to use a part of his soul and fuse it with Hades through the use of his own Divinity, the Creation Skill, and the Synthesis Spell he had just created.
The Synthesis Spell was something simple, Frank simply used magic to combine things, makings something new.
Check "The Protagnist's Sister Is Actually the Strongest"!