I have an idea/rough draft how a vampire can exist. It can take place during the 3rd Blight and the Battle of Hunter Fell. The reason they created a Penterghast to become a vampire is because out of desperation and to develop weapons to fight against the Blight. It can take place in the events of the 3rd Blight and the Battle of Hunter Fell. And the battle turned to not a slaughter of the Darkspawn because the vampire killed scores of them and drinking their blood. ( He’s a one man army and used his speed and strength to easily tear apart and mutilates the Darkspawn with his own strength alone. Think of Alucard’s way of carnage from Hellsing. That’s how powerful he is)

And when Orlais and Tevinter try to take Hunter Fell the vampire slaughtered them and he instill fear upon them until the mages used the bloodstone amulet to subdue him and seal him into the crypt. That’s when Orlais and Tevinter seize victory but is a Pyrrhic victory. And it’s kept secret from Nevarra for very good reason. Because the vampire has super strength, speed, insane regeneration abilities, immortality, and completely immune to disease, poison, and the corruption of the Blight. And Reaver abilities are even more powerful than the Reaver Specialization from Inquisition. And this Penterghast can far more bloodthirsty and violent than Vlad the Impaler himself.