The Night Mother is the spiritual leader of the Dark Brotherhood. It is the Night Mother who chooses the Listener when the previous one dies and it is the Night Mother who listens to the prayers of people who have performed the Black Sacrament and passes on their names and locations to the Listener.
Historically, the Night Mother was a Dunmer woman who was blessed with five children by Sithis himself, by mystical means. As a twisted act of worship, she slew all five of them to send them to Sithis to dwell in the Void. For this, she was lynched by the townspeople. As a spirit entity, she is imperious, cruel, very farsighted and conniving. She is the only direct link between Sithis and his servants in the mortal realm.
Black Sacrament
The Black Sacrament is a ritual performed by people wishing to have someone killed. The ritual consists of an effigy of the intended victim, formed from actual body parts including a skeleton, flesh and a heart. The person performing the ritual must then stab the effigy repeatedly with a dagger rubbed with the petals of the Nightshade plant while intoning "Sweet Mother, sweet Mother, send your child unto me, for the sins of the unworthy must be baptized in blood and fear."
Without the Black Sacrament, the Brotherhood would be forced to use more conventional ways to obtain information on contracts. This method typically offers inferior coverage and the Brotherhood had to struggle to maintain its reputation.