Nidhogg

Nidhogg (Nithhogg in the Netflix subtitles) is a giant serpent/dragon from norse mythology. The dragon lives in Nithafjoll (Niðafjöll) or "dark mountains" and eats the roots of Yggdrasil, the world tree.

Nidhogg is mentioned in the second episode of season 1, The Escape, by Orm in a recitation of a poem (from the "Völuspá") at Norheim's "Odes and Poetry" Club.

Orm's Poem
Quote: "From below the dragon dark comes forth, Nithhogg flying from Nithafjoll; The bodies of men on his wings he bears, The serpent bright: but now must I sink."

Orm/Bellows' translation