Major plot hole with Heavensward ending (Spoilers, duh)

[Allyrion]

I have to object to the notion of Nidhogg being a primal. Primals typically require prayers, crystals, and some other possible power. Nidhogg has no prayers or crystals. Not to mention that Nidhogg simply existing isn't draining aether. Dragons are a complicated bunch.

Also, please show me where you got the idea of Nidhogg possibly consorting with the Ascians because I really am not seeing it beyond a "really ticked off dragon".

I am talking about what possesed Estinien, not the normal Nidhogg.
I'm not saying it's a Primal for sure, just that there are hints that it is. Even if not, there are more hints that what possesses Estinien is an embodiment of his rage rather that the dragon himself.

The conversation between Estinien and the Nidhogg he sees in the eye is:

Nidhogg: Thou hadst done well to resist mine influence, bathed in my power and blood as thou wert
Nidhogg: Alas, in thine anticipation of comfort, thou hast lowered thy guard!
Nidhogg: The keening of my fallen kindred.. their smoldering desire for vengeance..
Nidhogg: Mine eyes have partaken of a thousand years of pain - a pain I will bestow upon thee.
Nidhogg: Drink deep of my rage, mortal...AND BECOME ME!

The desire for vengeance functions as a wish, the eyes themselves are a known aether source.
The Archbishop says the untouched eye they had hidden was almost limitless.
Also, Lahabrea got sort of eaten by the eye-sword.
The fact that this Nidhogg echos the "Become me" of the Ascians could be a hint with these in mind.

Even so, yea that's all speculation and I stated it as such.
It's just interesting the things Nidhogg is talking about to Estinien is the power of the eye and the desire for vengeance.
We know the eye's power is corrupted aether. That makes a recipe for a primal.
Not to mention we're dealing with this whole person transforming into a primal a lot now.

If he is a Primal or not is tin-foil hat worthy. I won't argue he definitely is
That said, that doesn't mean he's the real Nidhogg.

The times they've talked about him, they said Estinien is a vessel for Nidhogg's hate, not Nidhogg himself.
It's almost like they avoided ever saying directly saying Nidhogg is reborn and took Estinien's body.

Simply, it doesn't seem like you would say Estinien is "claimed as a vessel for the great wyrm's rage" instead of "the great wyrm" or even just "the great warm in his rage".

So yea, I think the new Nidhogg is his hate given form.
Though admittedly, that's not much different than the old Nidhogg.
That's a strange type of being then, so if it's a primal....well there's some hints it might be, but they're not too substantive.

More specifically

Primals typically require prayers, crystals, and some other possible power. Nidhogg has no prayers or crystals. Not to mention that Nidhogg simply existing isn't draining aether.

We don't know anything of the new Nidhogg, and that's what I'm talking about.
He just flies away. We don't know if he's draining Aether or not.

Regardless, like I said above, we have the recipe which he mentions himself.
The power of the Eyes and the wish for vengeance from his brood.
We also know the Archbishop was able to stay in his King Thordan form because the Eye had a "limitless supply of aether" to sustain him.
Perhaps an exaggeration but if the new Nidhogg is a Primal (big IF of course) then he will not need to drain Aether for a while since he has the eyes, one being very saturated with Aether.