In Curse Of Strahd, Zrin-Hala's sarcophagus was encountered by the Heralds of Dawn. He had little power to contact the adventurers as he had recently used his abilities on another champion, and had failed to get them to break his prison.
An ageless Dark Power native to the Plane of Shadow, Zrin-Hala was once a higher being who preyed on souls within the veil of the Deepmists. In the Pre-Unified Age, he was locked into an amber sarcophagus by high mages of the Azure Paragon, then was stored in the Amber Temple alongside other Dark Powers. Claiming the title "The Howling Storm," Zrin-Hala asserted to have ultimate power over lightning and wind Spellcraft, but was only able to influence the realm beyond his tomb with minor magic.
As his powers waxed and waned during the Barovian Calender Timeline, he contacted adventurers to harvest soul energy from Barovians. Like all Dark Powers, he had methods of feasting on souls departing mortal's bodies, and in exchange for magical power, would request that his champions destroy their sarcophagus. However, he experienced few successes, as souls leaving Barovia were nearly always consumed by Vampyr, and his champions were killed by the many horrors of the valley.