The Scheming Servant is a Nord warrior who may appear in Understone Keep in Markarth during the quest The Dark Brotherhood Forever. He will only be enabled if the Night Mother picks him as the contract giver.
He will be chosen randomly from a list of ten people who performed the Black Sacrament and want someone killed. When approached, he will choose between a total of ten possible targets, hand you a leveled amount of gold and blindly expect you to carry out the task.
He shares his dialogue with the ten other potential quest givers; see this section for details.