Arch-Curate Vyrthur is one of the last living Snow Elves, alongside his brother Knight-Paladin Gelebor, and is the secondary antagonist of the Dawnguard storyline. He is also a vampire, and believes that Auri-El is to blame for allowing him to become infected.
He believed that Auri-El had turned his back on him, as he not been protected from the affliction. In an act of revenge, Vyrthur created the Tyranny of the Sun prophecy, the fulfilment of which would weaken Auri-El's influence on Nirn. All he needed to realize the prophecy was the blood of a Daughter of Coldharbour and Auriel's Bow.
At some point in the First Era, the Chantry of Auri-El was attacked by the twisted remnants of his extinct race - the Falmer. Several stormed the Inner Sanctum, but Vyrthur evidently defeated the intruders, freezing them solid within casings of ice. 
In order to obtain Auriel's Bow, the Dragonborn is tasked by Knight-Paladin Gelebor to fill the Initiate's Ewer at five of Auriel's Wayshrines, in order to unlock the Inner Sanctum where Vyrthur awaits with the bow in his possession.
After entering Auriel's Chapel, he can be found seated on a throne behind a magical wall of ice. Approaching him will begin a short dialogue, admitting that he's been waiting all this time for someone to bring him the final component to him, namely Serana. Afterwards, he unleashes swarms of frozen Falmer and Chaurus upon the Dragonborn and Serana.
Once the swarms have been defeated, Vyrthur will destroy the entire roof of the chapel in a fit of rage and retreat out to a grand balcony overlooking the Forgotten Vale. When chased and confronted, Serana will initiate the following dialogue:
Arch-Curate Vyrthur "How dare you. I was the Arch-Curate of Auri-El, girl. I had the ears of a god!"
Serana: "Until the "Betrayed" corrupted you. Yes, yes. We've heard this sad story"
Arch-Curate Vyrthur: "Gelebor and his kind are easily manipulated fools. Look into my eyes, Serana. You tell me what I am."
Serana: "You're... you're a vampire? But Auriel should have protected you..."
Arch-Curate Vyrthur: "The moment I was infected by one of my own Initiates, Auri-El turned his back on me. I swore I'd have my revenge, no matter what the cost."
Serana: "You want to take revenge... on a god?"
Arch-Curate Vyrthur: "Auri-El himself may have been beyond my reach, but his influence on our world wasn't. All I needed was the blood of a vampire and his own weapon, Auriel's Bow."
Serana: "The blood of a vampire... Auriel's Bow... It... it was you? You created that prophecy?"
Arch-Curate Vyrthur: "A prophecy that lacked a single, final ingredient... the blood of a pure vampire. The blood of a Daughter of Coldharbour."
Serana: "You were waiting... all this time for someone with my blood to come along. Well, too bad for you... I intend on keeping it. Let's see if your blood has any power to it!"
After this exchange he will attack the Dragonborn. Upon his death his final words are "Farewell."
In battle, he uses ice magic that the Snow Elves were famed for. Spells such as Icy Spear and Frost Cloak, in addition to a unique variant of Vampiric Drain that also drains stamina, make up the core of his arsenal. He also typically summons a Frost Atronach at the start of the battle. Once his magicka runs dry, he switches to his frost-enchanted Elven Dagger.
Arch-Curate Vyrthur wears the only obtainable set of Ancient Falmer Armor. It can be taken from his corpse once he is killed.
- Despite being a Snow Elf, his blood cannot be used in place of Falmer blood in the quest Discerning the Transmundane. This is likely because his blood is tainted by vampirism, or because the feature had no plans to be added.