now this made me think of something good by using this quote as a reference and this spell from sword art online this would be almost like a werewolf form but with greater power more like the vampire lord transformation from skyrim
Metamorphosis
Once the incantation is completed, the spell turns the caster into a
monster. The shape of the monster that the player is transformed into depends randomly on the player's combat skills. However, this spell is not popular, as the end result is usually a weak monster with no change in stats, thus it is usually not viable in actual combat.
Beings of strange form and appearance made by the gods in ancient times. Favored with great strength and intellect, the Espers knew power far beyond that of men, but their power made them proud, and at length they sought to challenge the gods. Seeing this, the gods were angered and struck down their blessed children, and binding their souls and flesh with the Glyph of the Beast, they stole their freedom for all eternity. Now they are bound to live only when summoned by their Glyph, to serve whosoever called them forth.