Vampyr
The Dark Power that gave Count Strahd the curse of Vampirism. It forced Strahd to kill his ally and brother in order to win Tatyana's affection only to laugh at Stahd after she fell to her death. According to the Tome of Strahd, it seems that Vampyr will bring Strahd back from the dead whenever he is destroyed.
There may be more info at the Amber Temple
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Dark Power
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