The Dark Brotherhood is a guild of killers that operate throughout Tamriel. While their business is undoubtedly illegal, their presence is usually woefully overlooked. Whenever an individual with power takes an interest in their dealings, the Brotherhood counts upon bribery, blackmail, intimidation, and murder to keep their activities covert. They are bitter rivals of the Morag Tong. The Brotherhood is also known for worshiping the Dread Father Sithis (personified chaos, also known as the Void). Dark Sisters are accepted into the Brotherhood as equals of Dark Brothers (though members of either gender are referred to as Brothers).

The Fire Tenets is a set of rules that must be abided by most of the members of the Dark Brotherhood. However, the Five Tenets do not restrict the ranks of Silencer and above. The Five Tenets are as follows.

• Tenet 1: Never dishonor the Night Mother. To do so is to invoke the Wrath of Sithis.
• Tenet 2: Never betray the Dark Brotherhood or its secrets. To do so is to invoke the Wrath of Sithis.
• Tenet 3: Never disobey or refuse to carry out an order from a Dark Brotherhood superior. To do so is to invoke the Wrath of Sithis.
• Tenet 4: Never steal the possessions of a Dark Brother or Dark Sister. To do so is to invoke the Wrath of Sithis.
• Tenet 5: Never kill a Dark Brother or Dark Sister. To do so is to invoke the Wrath of Sithis.

The Wrath of Sithis is a wraith-like spirit that will confront the tenets-breaker assassin in their sleep. If the assassin could defeat the spirit, the Brotherhood would accept them again in the guild. However, they would be exiled forever if they broke the tenets three times. They would never again have a chance to rejoin the Brotherhood.

In distinction to the Morag Tong's reverence of the Daedric Prince Mephala, the Brotherhood revere the Void – Sithis. However, they see their murders as more of a business, viewing their duty through the lens of economics rather than religion and honor. They are thus oftentimes viewed as being more practical than the Morag Tong. These distinctions have yielded animosity between the two organizations.

The Night Mother has always overseen the Dark Brotherhood. Some tales suggest that the Night Mother is the ghost of the first, who became the wife of Sithis and murdered their five children in his honor. Others say that the title is merely an honorific given to the current leader. Some even believe her to be none other than Mephala, who is typically considered to be the "Patroness of the Dark Brotherhood." 

Nevertheless, the most famous and notorious method of obtaining the services of the Brotherhood is by performing the Black Sacrament. This profane ritual purportedly allows those who execute it to be heard directly by the Night Mother. The ritual involves stabbing several body parts (including a heart, skull, bones, and flesh) within a circle of candles using a dagger rubbed with the petals of a Nightshade plant while whispering the plea, "Sweet Mother, sweet Mother, send your child unto me, for the sins of the unworthy must be baptized in blood and fear." 

Next in the command hierarchy is the shadowy Black Hand, composed of a Listener and four Speakers. The Listener, of course, listens to the Night Mother and relays her assignments to the Speakers. They then dispatch a Brother to make specific arrangements and carry out the murder. Their identities are kept well hidden, even within the organization.

The Brotherhood's origins are connected with the Morag Tong, though how is a topic still debated by scholars. However, it is widely believed that in the year 1E 2920, the Morag Tong assassinated Emperor Reman III, likely at the behest of Versidue-Shaie, his successor. Then, in 2E 324, they assassinated Versidue-Shaie himself, setting off a frenzy among the nobles across Tamriel. As a result, the Morag Tong was soon outlawed across the continent.