In Search For The Silver Dragon, Vampyr was contacted by Zandros, who teleported into Barovia without authorization and brought back three powerful souls. In return, Vampyr allowed Zandros to see Strahd, and separated him from the other interloping adventurers. They were left near-dead on side of Mount Ghakis, but managed to recover, and formed an adventuring party to oppose the Dark Power.
An ageless Dark Power native to the Umbral Realms, Vampyr was once a higher being who preyed on souls throughout the mists of the Inner Planes. They were locked away in the Balinok Mountains, encased in the Amber Temple in a sarcophagus of amber alongside many other great forces. Unlike the other trapped Dark Powers, however, Vampyr was clever enough to release themselves. When Strahd von Zarovich turned to them in a moment of rage and weakness, Vampyr formed a greater pact with him, turning the Count into a Vampire Lord and providing him with great powers of darkness and undeath.
In turn, Strahd released Vampyr from their imprisonment from their sarcophagus, locked next to the sarcophagi of Khirad and Delban. Vampyr gave Strahd not only great personal power but also complete ownership of a Demirealm, choking the land with mists that pulled the Realm from the Prime Material into the Plane of Shadow. Now free to roam the mists around the newly-created Barovia, they began to sustain themselves by feasting on the souls of Barovians, and used this power to rip Barovia from the Prime Material into the Umbral.
They played with Strahd, ensuring that the man was flipped between continual states of intense desire and manic tragedy, and used him to satisfy the eternal hunger that all Dark Powers possessed. When their hunger could not be satiated with death, such as through ageless beings like The Abbot or Orlev ben Khazan, Vampyr was incensed. They would use their lingering presence in the Amber Temple to rot the memories of these creatures, as punishment for their "resistance" against their lord's desires.