Lucien Lachance is an imperial assassin and speaker of the Black Hand living at Fort Farragut. He is the one who invits you to join the Dark Brotherhood, gives you your first contract, sends you to the Cheydinhal Sanctuary, gives you your contracts after your rise to the rank of Silencer, and who's death is in part, by your misguidance.
After commiting a murder and then sleeping in a 'secure place' Lucien will appear and invite you to join the Dark Brotherhood by fist killing Rufio at the Inn of Ill Omen. He then sends you to the Cheydinhal Santuary where the rest of your contracts will be given until you are able to recieve contracts from the Black Hand itself. The first contract given by the Black Hand sends you back to Lucien who you now know is living at Fort Farragut. He tells you there is a traitor among the Dark Brotherhood at the Cheydinhal Sanctuary who was there before you joined but they don't know who it is and so a 'Purification' must be carried out on the sanctuary. Your told to kill everyone at the sanctuary and as a reward for doing so are 'loaned' Shadowmere, Lucien's immortal horse.
You will then pick up your next contracts and rewards from 'dead drops' only meeting Lucien if he deems nessacary. After completeing the seventh dead drop Lucien Lachance appears enraged, saying you have been systematically killing off other members of the Black Hand and the Dark Brotherhood now suspects him of being a traitor. After relizing someone had been switching the dead drops on Lucien you must prove his innocence and find the real traitor among the Black Hand.
At the eighth dead drop you find someone named Enilroth switching them but informs you he was payed to do so by someone staying at the light house. After recovering the diary of a Bland Hand member who had been a member of the Dark Brotherhood and had a strong hatred for Lucien since childhood for killing his mother and returing to Applewatch with it where Lucien said to meet him but you find he has already been killed by the remaining Black Hand.
The Black Hand informs you that they don't blame you for the killings and that the traitor Lucien Lachance refused to admit to betraying the Dark Brotherhood even upon his dying breath and only when the Night Mother informs them of the 'true' traitor due they see there error.