They have an insane ideal and follow the instructions given to them by a dead woman who was believed to be the bride of a literal god and birthed his children.
They are by far the most cultist lunatics in the series. RELATED: 10 Games to Play If You Loved Assassin's Creed: Valhalla While most other factions follow the Daedra, the Brotherhood follows the Dark Lord Sithis instead, using the soul of his wife, The Night Mother, to guide their actions.
Introduced in Oblivion and carried over into Skyrim, the Dark Brotherhood is one of the most engaging side quests of the series. If there is something that will divide the Elder Scrolls player base, it's whether or not the Dark Brotherhood is destroyed or joined.