Following a Lead
Oblivion: Dark Brotherhood Quest
Immediately after you kill Ungolim for the quest A Kiss Before Dying, Lucien Lachance will confront you. You've betrayed the Brotherhood, and he must now kill you, he says.
Seeing the look of confusion on your face, though, he realizes that you have absolutely no idea what he's talking about. So, he explains:
You haven't been answering your dead drops. Your first couple of drops went well, but after that you began murdering members of the Black Hand! Apparently, somebody has been leaving you false orders. (If you look at the Dead Drop orders you've received up until now, you'll notice that the style of the letters changes with the ones you received in Broken Vows.
Since Lucien is also on the line (the Black Hand suspects that he has been directing you), he sends you to Anvil to wait for the next dead drop, and apprehend the traitor who is leaving the fake orders.
Head to Anvil and follow the marker to a barrel where the next orders are expected. Go behind the barrel and "sneak" until Enilroth leaves the next orders in the barrel. Talk to him, and he'll quickly deny everything. He tells you that he's been paid to leave the orders there, and that the one who sent him lives in the lighthouse.
You'll need access to the cellar below the lighthouse to find the traitor's lair. Go up into the lighthouse to find the keeper. You can either kill him for the key, or ask him nicely. Tell him the Dark Brotherhood needs it, and he'll fearfully hand it over.
Enter the creepy cellar below the lighthouse. You'll find a crazed dog inside, but other than that it's empty (apart from a few bodies). In the last room of the cellar, you'll find a creepy, decapitated "Mother's Head" and a book, the Traitor's Journal. Read it.
Head back to Applewatch Farm to meet up with Lucien. Once you enter, though, you'll find the rest of the Black Hand.
Lucien is dead. (That's him hanging from the ceiling.) The Black Hand is satisfied that they've removed the traitor, and promotes you to Speaker in Lucien's place. However, since the Listener of the Dark Brotherhood was killed (by you), you must travel with them to visit the Night Mother.
If you're up for a bit of fun, you can drop the Mother's Head from the cellar on the floor around the other Speakers. One of them will react to it in a suspicious way, although it doesn't change how the rest of the quest line plays out.
Speak to Arquen to progress to the next quest: Honor thy Mother.
Speaker, Dark Brotherhood
Reached Speaker rank in the Dark Brotherhood
10 