In Curse Of Strahd, Ten-Zras took notice of Mormeros Fayan, who he deemed to be a suitable "candidate" as his champion. Mormeros, already a Warlock patron of the Archdevil Dharus, was interested in hearing how Ten-Zras could help him to supplant Strahd von Zarovich and Vampyr as the new Lord of Barovia, and frequently contacted the Dark Power. Eventually, while trying to prevent Anastrasya Karelova from alerting Strahd von Zarovich of his indiscretion and violence towards her, Mormeros made a full pact with Ten-Zras, violating his Infernal contract and ensuring he could never go where Dharus could find him.
This new, powerful relationship did not last long, however. Strahd attacked the group outside their new base in Argynvostholt, and toyed with the group, slowly isolating them and draining them of their magic. Realizing that they would all be hunted down, Mormeros decided to save his friends. He fully gave himself over to Ten-Zras in exchange for as much power as possible, and the resulting surge of dark magic immediately killed him. However, his gambit worked; a shadowy form of Ten-Zras overcame Mormeros, lifted by seven faceless Undead, and chased Strahd off into the sky then down the mountain.
After this brief explosion of power, Ten-Zras piloted his new body to the Amber Temple by foot, intent on destroying the sarcophagus holding him and releasing him into the Plane of Shadow. Though he had repelled Strahd, however, he was not as powerful as the Vampire lord, who hounded Mormeros' body as he traveled. Weakened, he made his way to the temple, where Strahd commanded Neferon to allow him in unaccosted. When the Kingmaker was only meters away from his sarcophagus, Strahd intervened and dispelled Ten-Zras' possession on the warlock, leaving his dead body in the vault and surrounded by the Wights that lifted him.