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Consider that normal sorcery is made using the power of "soul" and the dark magic is made using the most human aspects of mentality(check the descriptions of dark magic). I think the abyss serves to show what an Age of Dark would look like, considering it is literal Ground Zero of the Dark Soul Genesis or whatever. wouldn't he technically be an arch-angel? You can confirm this theory by looking at Ornstein's Soul description. Artorias was a human who was granted a powerful soul by Gwyn, thus turning him into.
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