Sithis is the representation of the state of chaos and serpentine embodyment of entropy. He is the patron of the Dark Brotherhood and husband to the Night Mother who bore him five children, which she killed and sent into the void.
He is not a daedra and does not dwell in Oblivion but was the huge black nothingness that existed before creation. Sithis is often depicted as a skeleton but is shapeless and as infinite as the void. His relationships with other spirits are mysterious and varying. He is referred to by some texts and legends as the child of Mephala and is thought of as an equal but opposing force to Anuiel.
The Dark Brotherhood runs like a business fulfilling contracts but will always demand blood in the name of Sithis and those within the Brotherhood who break one of the 'five tenets' must face and defeat the 'Wrath of Sithis' to regain entry. The souls of those slain by the Brotherhood are sent into the void to Sisthis himself. The sentient Hist trees acknowledge Sithis and Argonians born under the sign of the shadow are taken to the Dark Brotherhood to train as shadowscale assassins and serve Sithis. All shadowscales are unable to kill other shadowscales and can never leave the Brotherhood under penalty of death.