Originally Posted by
Cilia
Sort of...?
Let's think back to Hraesvelgr and Nidhogg sharing eyes (basically). Nidhogg was on death's doorstep before Hraesvelgr gave him one of his eyes, but that didn't color Nidhogg's psyche at all. Then Nidhogg's corporeal body disappeared after Estinien gouged out said eye. Thus, clearly dragons need an eye - any dragon's eye - to maintain a corporeal form. Nidhogg had none, and vanished from the world accordingly. Except his eyes.
Think of it like blood or vital organs. You need them to live, but they don't have to be yours - any compatible ones will do. If you don't have them, you die, end of story. Your organs, even though you died, can still be used to save another person's life by transplanting them in, though; you'll remain dead. Now, if your will / emotions lingered in your organs and was able to possess people, you could pull a Nidhogg.
Back in fantasy land: Nidhogg had (or once had) emotional settings besides "hate and rage," while that's all that lingered in his eyes after his death. Thus, Estinien-Nidhogg isn't Nidhogg reborn, but his lingering hatred possessing Estinien's body and somehow reconstituting Nidhogg's form.