The Five Tenets

The Dark Brotherhood was originally bound by the rules known as The Five Tenets. Astrid has since revoked this code and chooses to do whatever she pleases, as "her word is law."

Summary
This book details the core values of the Dark Brotherhood. They have since been abandoned in Skyrim.

Known locations

 * Dark Brotherhood Sanctuary
 * Fort Farragut
 * Dawnstar Sanctuary
 * Carried by Vicente Valtieri

Content
''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.''

Trivia

 * Curiously, the Tenets seem to have been created by Sithis. They say that killing a Dark Brother or Sister will invoke the Wrath of Sithis, so, if these were created by Sithis, Astrid would technically be violating her master's rules by revoking the Tenets.
 * Interestingly enough, Astrid breaks all the Tenets, save for Tenet 4 during the Dark Brotherhood quests.
 * Tenet 1: She dishonors the night mother by her reaction and attitude to having the Night Mother's corpse reside in the Falkreath Sanctuary.
 * Tenets 2 and 5: She betrays the Dark Brotherhood, and directly leads to the deaths of most of the members.
 * Tenet 3: She initially disobeys and refuses to carry out the Night Mother‘s wishes for the Dragonborn to seek out Amaund Motierre at Volunruud.
 * One could make the case that perhaps she did in fact invoke the wrath of Sithis.