Given the location (Ishgard), the big bad in question could very easily be a Dravarian considering the whole Dragonsong War effort. They're big, powerful, tend to require a lot of effort to bring down, so having people hunting it and/or requiring help to do so would certainly make sense. And even with the whole resolution of the War at the end of Heavensward, it's still stated that those of Nidhogg's brood are still out there causing problems. This could be one of them!
Alternately, a powerful Voidsent is always a good fallback for power monster. Either summoned, snuck its way across the Void somehow, or a previously bound one that has been released from its servitude/prison. While not part of Black Magic itself, summoning Voidsent was a big part of Mhachi proceedings in bigger and bigger bids for power... so that could tie the Black Mage into things.
Alternately, a powerful Voidsent is always a good fallback for power monster. Either summoned, snuck its way across the Void somehow, or a previously bound one that has been released from its servitude/prison. While not part of Black Magic itself, summoning Voidsent was a big part of Mhachi proceedings in bigger and bigger bids for power... so that could tie the Black Mage into things.
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