"Death"

Death was the name Strahd gave to the dark entity that he seemingly made a pact with for his Heart's Desire. According to Strahd's description, it was a taunting and tempting entity, almost a force, that mimicked the voices of his friends and family members. To seal the pact, Strahd murdered Alek Gwilym and Sergei von Zarovich and drink their blood to transform fully into an immortal vampire.

After Tatyana Federovna apparent suicide and disappearance into the Mists, Death taunted Strahd, but later it seemed to disappear, as its voices to Strahd were replaced with only silence.

Death's true nature is only hinted at by Strahd's memoirs, and the veracity of these descriptions are unknown. As it is, Death remains largely a mystery like the Dark Powers.

It should be noted that "Death" is not the same entity as Lowellyn Dachine, the darklord of Necropolis bearing the same name.