Vampyr

Vampyr is the father of vampires whose cruel presence first granted the form of immortality to the Mer in his champion Count Strahd Von Zarovich. Vampyr was trapped in amber when they met but was released by the shattering of his prison. Under his command Count Strahd Von Zarovich was sealed within Barovia which was enclosed in a wall of magical fog that prevented any soul from entering or leaving without the Count's permission.
In life Vampyr was an archdevil who was discontent with Asmodeus' rule. Governing over the plane of gluttony Vampyr traded souls and knowledge with Demons and Devils alike in order to find a means to cross the realms. Asmodeus supported his efforts at first curious to see if he might bring power back to the Black Gate but began opposing his work towards its end when it became clear that he merely wished to solidify his own power. When Vampyr began his ritual Asmodeus learned that all the knowledge and treasures he had gathered would be lost and consumed and sent his forces to destroy the archdevil but it was too late. As Vampyr moved through the void his form dissipated and collapsed leaving only a shadow of his former self. Yet with Asmodeus' tampering's he found a means of preserving life and with it a means of regaining his own power.
When a vampire turned by Vampyr's champion, or his champion themselves, feasts on blood Vampyr gains power consuming their soul as his chosen consume their blood. He is the most famed of the Powers That Be due to the significance of Barovia to early Falaran storytelling.