How does Nidhogg's eyes grant powers to an Azure Dragoon?

[Bright-Flower]

The thing that confuses me about Nidhogg 2.0 when he possesses EStinien is that it seems to just be a shade, and Midgarsormr doesn't seem to think it's actually Nidhogg...but then after the fight, it looks like NIdhogg's soul is 'freed' to the afterlife? Seems like if it was just a shade that wouldn't happen?

[RenewalXVII]

The thing that confuses me about Nidhogg 2.0 when he possesses EStinien is that it seems to just be a shade, and Midgarsormr doesn't seem to think it's actually Nidhogg...but then after the fight, it looks like NIdhogg's soul is 'freed' to the afterlife? Seems like if it was just a shade that wouldn't happen?

Less his soul, and more the malevolent will that clung to the Eyes' aether. It's honestly more of an academic difference, but all that remained of Nidhogg in the Eyes was his hate and rage, the strongest emotions he had left--the shade had the shape of a soul perhaps, but not its fullness. In theory, the true Nidhogg that was slain at the Aery might have eventually let go, even if that's unlikely of the First Brood: the capability existed, even if it never happened. The shade literally would never be anything more than Nidhogg's malevolence given flesh through Estinien.