In Curse Of Strahd, Sykane's long-nameless Deathlock champion was killed by the Heralds of Dawn, and Tobias Villie Broadnose Cogsmith uncovered his journal. This lead to him being contacted by Sykane; the Dark Power offered Tobias power over souls in exchange for patronage, but he resisted making a pact, feeling he could not agree to their Warlock terms.
An agelesss Dark Power native to the Umbral Realms, Sykane was once a higher being who preyed on souls within the veil of the Deepmists. A being composed of thousands of souls they had consumed, they were locked away in the dread domain of Barovia, encased in the Amber Temple in a sarcophagus of amber. Adopting the title "The Soul Hungerer," they claimed to have ultimate power to manipulate mortal souls, but were only able to influence the realm 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.