However, this history conceals Nidhogg's hunger to avenge the murder of his brood-sister, Ratatoskr. Over the past millennium, he has awakened on eight occasions, and each time the Holy See has tasted his murderous wrath. The dragon was slain at the Aery by the Warrior of Light and Estinien, the Azure Dragoon.
Patch 3.3: Revenge of the Horde
However, within the boundless aether of his eyes, Nidhogg's hatred lived on, and seized control of Estinien's flesh so as to fight to the bitter end.