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