In the Castlevania series of games, Death works as Dracula's servant?
Why is this? Dracula started off life as a human, did he coerce Death during his life or after the start of his unlife?
What power does he have over Death?
The Castlevania timeline is vague and often retconned, but at least one game has dealt with this explicitly.
Castlevania: Lament of Innocence was set in 1094, and tells the story of the birth of Dracula. In it there are two stones, the Ebony Stone and the Crimson Stone, the latter of which gives the owner eternal youth - and also turns them into a powerful vampire. Eternal youth is a metaphorical "mastery of death", and according to Death in the game, the stone provides a literal one as well. He swears fealty to whoever controls it.
In the newest timeline, Lords of Shadow, the relationship between Dracula and Death is more complicated. Death is a servant of Satan, who is trying to kill Dracula, but Satan betrays and kills Death, and upon returning he allies (maybe? The ending is ambiguous) with Dracula.