I'm with Hitei on this, 200%.Why would "merely a blade" be swearing vengeance against the Old Gods "for what was done to [her] long ago"?
Why would it continually use "us" and "we" when referring directly to the Old Gods and show such contempt for the other Old Gods and most other things?
Why would Blizzard conveniently leave her history unconfirmed and just a bunch of rumors and theories, one of which is that she is the "remains of a forgotten Old God"? If she was just a blade infused with power?
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The implication is way too strong and consistent. She is what is left of a 5th Old God.