Black Sacrament

The  is a ritual performed by one who wishes to contact the Dark Brotherhood, usually in the hopes of having a murder-contract satisfied by one of their assassins. The Night Mother hears the pleas of the invoker and translates the information to her Listener. The Listener would then relay this information to Speakers of the Black Hand. From there, the head of the nearest Sanctuary sends an operative to answer the call.

Silences in the brotherhood
After the Dark Brotherhood of Cyrodiil was destroyed and the Black Hand dismantled, the Night Mother no longer had Listeners. Instead, assassins of the Dark Brotherhood heard of the Black Sacrament through local rumors.

Return of the Listener
After Cicero brought the Night Mother from Bravil to the Falkreath sanctuary, she began translating prayers she had heard by invokers of the sacraments.

Dragonborn
Had the Dragonborn joined the Dark Brotherhood, he or she became Listener and received messages of those who practiced the ritual.

The ritual
In order to perform the Black Sacrament, the invoker must create an effigy of the intended victim, assembled from actual body parts, including a heart, skull, bones, and flesh. They must then encircle that effigy with candles. The ritual itself must then commence. The invoker must proceed to stab the effigy repeatedly with a dagger rubbed with petals of a Nightshade plant while whispering this plea: "Sweet Mother, sweet Mother, send your child unto me, for the sins of the unworthy must be baptized in blood and fear."

Hammerfell

 * An Imperial woman named Camilla performed the Black Sacrament, hoping to have a contract fulfilled against an unknown Cyrodilic citizen.

Cyrodiil

 * Talos District resident Claudius Arcadia was arrested by the Imperial Watch, after performing the Black Sacrament to have a contract fulfilled against a man named Rufio, who raped his wife. The Hero of Kvatch may have murdered Rufio, thus gaining entrance into the Dark Brotherhood.
 * Francois Motierre performed the dark ritual on his mother, hoping the Dark Brotherhood would assist him in feigning his own death and evade assassins to whom he owed money.

Skyrim

 * Aventus Aretino performed the Black Sacrament, hoping to have a contract fulfilled against the headmistress of Honorhall Orphanage, Grelod the Kind. Helping Aventus leads to the quest Innocence Lost.
 * Maven Black-Briar, although it is unclear whom she intended to have assassinated, attempted the sacrament in Black-Briar Manor in Riften. The ritual's effigy is hidden behind a locked door in the basement, accompanied by a note addressed to Astrid. In the note, Maven describes her frustration that no assassin of the brotherhood answered her prayer. There are also a pair of letters, one found in the Bannered Mare the other in Black-briar Manor that suggest a disagreement exists between Maven and a person named Christophe Bartlet. Christophe's disappearance, coupled with the sinister wording of Maven's letter, suggests he was the intended target of the Black Sacrament.
 * Astrid performed the Black Sacrament on herself so the Dragonborn could track her down. She wanted to apologize before asking the Dragonborn to kill her.
 * At Volunruud, Amaund Motierre performed the Black Sacrament, hoping to have a murder-contract against Emperor Titus Mede II fulfilled.