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 you have been tricked into systematically killing off members of the Black Hand the Dark Brotherhood suspects Lucien 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.
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.