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Frank willed it, as he created a spell that Hades was most likely going to teach him slowly, Undead Combination.
Alongside the spell he already knew the moment he got Death Magic, Undead Transformation, he acquired the ability to manipulate the dead spirits that were charmed by his Death Charm, and force them to possess either corpses or inanimate objects, becoming Undead-type monsters.
Afterward, the effect Undead Combination takes effect, combining the Dead Spirits and the corpses that would be used as their new bodies.
All of this happened in front of Frank's sight, as the Dead Spirits of the four beastmen that a few minutes ago hated him literally to death, now obeyed him happily, combining into a single ethereal mass and entering the four corpses simultaneously.
Check "The Protagnist's Sister Is Actually the Strongest"!