I agree with SannaR.

Judging by what Varshahn/Vitra says regarding Estinien, Estinien is as much Nidhogg as he is an elezan man. I think the fusion of their aether is what happened, and everything in game hints that much of the reason behind Estinien's incredible boost in power is because he's permanently infused by the aether of one of the first brood.

Regulators, interestingly, would never had to account for dragons and their unique physiology since the dragons only exist on the sundered Source. But I do wonder if a fighter tries to use a dragon like a feral soul in a future fight.