Crazy thought. What if its tied to the many 'sealed away' primals that Taimat mentioned. Remember in the Cutscene of the battle of Silvertear Lake, when the Airship blows up primals shoot out from the explosion.

What if there is a heap of primals sealed away underneath the lake and that's what Midgardsormr was guarding? Not sure how that would tie to Nidhogg though.