Don't Underestimate Her
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While other Vampires utilized a mix of many elements or abilities through their spells and skills, Agatheina simply used the very same Element she discovered and has been polishing since her beginnings as a Goddess.
Blood!
This element, which she was the very first in Genesis to harness, study, and evolve was her only and greatest weapon. And even if her Spells seemed simple, their effects were incredibly strong and lethal, a result of her refining their power and effects for literally millions of years.
If the years inside her Divine Realm could be taken into consideration, indeed, she had lived for so many years.
As the Demons surrounding her were blown up into bits and sliced apart in mere seconds, Agatheina quickly started draining their Demonic Blood, gathering even more power.