Amberser didn't know the limits of Aeolus's rebirth ability, he only knew that this youngster's ability to come back to life was outrageously strong.
Therefore, Amberser needed to find a way to deal with this problem first, otherwise he would just be wasting magic power.
True Polymorph was a powerful Ninth Circle Spell that could, in theory, permanently transform the target into another species. Normally, the effect of such a transformation was hard to achieve because the more powerful the creature, the greater its resistance to the spell.
Aeolus had the blessing of a Deity, making it extremely difficult to transform him.
So Amberser chose to kill him first, thinking that at the moment of beheading, his will would be weakened and he should be more susceptible to the spell's effects.
Amberser's plan was a success, the Ninth Circle Spell was cast smoothly, and Aeolus's body instantly turned into a crab the size of a fist.
