This sidequest is unlocked after starting the last main Dark Brotherhood quest, Where You Hang Your Enemy's Head. During your stay in Dawnstar Sanctuary you might be contacted by the Night Mother, asking you to hear her. Walk towards a coffin near the exit then (screen above). The game will be giving you here random contracts. Every time the Night Mother will tell you who the client is, and then you'll learn from him who your target is. Every successful mission is rewarded with gold of course.