Great Serpent

The Great Serpent was a being from Norse mythology, presumably involved with Ragnarok. It was believed that "the Great Serpent will rise from the sea and spew venom over all the earth." When Fenric planned to destroy humanity in 1943, he introduced Ingiger to Millington as the Great Serpent and planned to have him release poison into the waters. (TV: The Curse of Fenric)

Behind the Scenes
The Great Serpent is most likely analogous with who is meant to poison the sky before its final battle with Thor.