The Garlean Empire vs Omega

[Iscah]

No conflicting argument. He was not influenced permanently as far as we know or can tell, at least inasmuch as his personality is concerned. His rage was temporarily synchronized with Nidhogg's and he called upon him for more power in the A Realm Reborn job quests but managed to shake it off offscreen, and once possessed of both eyes of Nidhogg and covered in his aether-rich blood he was taken over by Nidhogg's lingering will but was freed from this by Alphinaud, the Warrior of Light, and the lingering wills of the Ysale and Haurchefant.

His changing attitude towards dragonkind is attributed to his experiences, rather than any aetheric influence. Anything else would be convolution of story and really isn't supported by the game's text.

While there is no indication that he is tempered, his experience with Nidhogg has left a deeper mark on him than you seem to recognise here. There are a number of suggestions in various places that he has absorbed some aspects of Nidhogg's personality as a consequence of being merged with him, because he felt Nidhogg's emotions as his own during that time - but lucidly, it seems, and with the ability to decide what to do with those emotions.

He directly felt Nidhogg's rage and grief at Ratatoskr's death, and so he himself grieves for Ratatoskr at a personal intensity, and it is at least implied if not stated that he "thinks like a dragon" to some degree now - not because Nidhogg made him do it but because it was a natural outcome of being so deeply exposed to Nidhogg's soul.

I don't have references for in-game quotes, but the side story From Azure Ashes covers the core of it.

For a time, Estinien’s soul had been merged with that of Nidhogg; the dread wyrm’s tumultuous storm of emotions comingling with his own. The rancor, the fury, the grief─he had felt it all. And the grief lingered even now. The gulf left behind by Ratatoskr’s passing was one he knew as well as any man or dragon.

[Zero-ELEC]

While there is no indication that he is tempered, his experience with Nidhogg has left a deeper mark on him than you seem to recognise here. There are a number of suggestions in various places that he has absorbed some aspects of Nidhogg's personality as a consequence of being merged with him, because he felt Nidhogg's emotions as his own during that time - but lucidly, it seems, and with the ability to decide what to do with those emotions.

He directly felt Nidhogg's rage and grief at Ratatoskr's death, and so he himself grieves for Ratatoskr at a personal intensity, and it is at least implied if not stated that he "thinks like a dragon" to some degree now - not because Nidhogg made him do it but because it was a natural outcome of being so deeply exposed to Nidhogg's soul.

I don't have references for in-game quotes, but the side story From Azure Ashes covers the core of it.

Right, and I'd put that in with "experiences" rather than any "aetheric influence". The man hosted the angry shade of a great dragon for the better part of three patches (however long that is), he remembers those feelings as if they were his, because they were, for a while. And when you feel as deep as Nidhogg did, feelings they don't go away that quickly, if ever. The feelings "lingering" is normal, as normal as being possessed can be.

Basically he gained empathy for the plight of the dragons after being Nidhogged up for a while, rather than being mind controlled. His personality is intact.

[Necrotica]

Right, and I'd put that in with "experiences" rather than any "aetheric influence". The man hosted the angry shade of a great dragon for the better part of three patches (however long that is), he remembers those feelings as if they were his, because they were, for a while. And when you feel as deep as Nidhogg did, feelings they don't go away that quickly, if ever. The feelings "lingering" is normal, as normal as being possessed can be.

Basically he gained empathy for the plight of the dragons after being Nidhogged up for a while, rather than being mind controlled. His personality is intact.

Traumatic memory induced brainwashing then.