As soon as you complete the hit on Ungolim Lucien Lachance in person comes and begins to threaten you. He says every hit you've made was a member of the Black hand (Silencer, Speaker, and now Listener) he quickly realizes that you have no idea what's going on and tells you that the surviving Black hand suspects he is the culprit. Turns out someone has been switching out his dead drop notes with their own. From now on you won't be taking orders from anyone—you'll be finding the traitor and bringing him to justice.
Following a Lead
Head to Anvil and the next drop point. You'll find a barrel near a statue pretty close to the main gate. Hang out for a while and you'll see a wood elf come and switch the notes. Speak with him and interrogate him as you see fit (threats are befitting of a Dark Brotherhood member). He'll tell you that a man in a robe and hood has been paying him to switch the notes, judging by his terror he's telling the truth. He tells you the man lives in the lighthouse cellar (what kind of lighthouse has a cellar anyway?) and the lighthouse keeper could give you the key.
Head to the Anvil Lighthouse and go inside. Speak with the lighthouse keeper and he'll give you the key after you threaten him. Head outside and go around the lighthouse until you see the cellar door.
Inside you'll find well pretty much what you'd expect from a dark brotherhood member dead bodies, a couple of dead dogs, and plenty of blood. Don't worry though he's not here. Go through the creepy lair and enter the last room you'll be attacked by a wolf. Kill it and enter the room. Here's where he becomes more insane then the usual Dark brother. There's a small shrine with his own mother's severed (and fairly rotten) head on it. You don't need to take the head, but you can anyway. Read his diary on the floor to see him narrate his entire life. It seems that Lucien Lachance was his father and he carried out a hit on his mother. Apparently, he's been rising through the ranks and is now a speaker. He's hoping to kill the Night Mother when she chooses a new Listener.
With all this information, head to the Draconis Farm where Lucien Lachance said he'd meet you. Enter the house and you'll find you're too late. The four other members of the Black hand are there and they have brutally killed and mutilated Lachance. Arquen will tell you that they need to speak to the Night Mother and that you are innocent, she then appoints you the rank of speaker (there have to be five fingers to the Black Hand) and tells you to come back between 12 and 3 AM. One of these four speakers is the traitor, but don't worry he'll reveal himself when you meet the night mother.
Honor Thy Mother
If you come back between 12 and 3 AM and speak with Arquen then you'll begin the final Dark Brotherhood quest. It turns out the Lucky Old Woman Ungolim (the old listener you killed) was actually the Night Mother's Tomb. The four speakers will pray to the night mother as you watch and the statue will reveal a passage to her tomb. Here the Night Mother reveals herself as a ghost and tells Arquen the traitor is still among them. Immediately Bellamont stabs and kills two of the other speakers (you can't stop this it's a cut scene) and rounds on you. Together you and Arquen have to kill him.
The Night Mother is pleased and promotes you to the position of Listener. She then bids you to rob her tomb. Take everything you can, because you're never coming back. Speak with her again and she will enchant your Blade of Woe so that it deals all kinds of damage. She then Teleports you and Arquel to the Cheydinal Sanctuary.
Now that you're a Listener and 90% of the dark brotherhood is dead you won't receive any more quests there are still some perks to being the boss though. You can get a weekly paycheck just for listening to the Night Mother and you get a selection of lackeys to choose from. Other then that there really isn't much to do.
Whispers of Death
Once a week you can return to the statue of the Night Mother and listen to her tell you the names of targets that people have prayed for. Do not worry as none of them are in Cyrodiil so you can't personally eliminate them.
Head back to the Cheydinal sanctuary and tell Arquen the names to heard. She'll send some new recruits to assassinate them and give you a cut of the money (200 gold the first time.) Remember you can do this only once a week so after a bit of adventuring see what day it is and listen to the new hits.
As Arquen works new members will populate the sanctuary. All of them naturally worship the ground you walk on and you can choose one of them to take with you as a lackey. There is a Khajiit with an axe, an elven archer, and a female (Breton?) spell caster. As assassins they are better then normal minions and can obey more commands (ex: follow me, but stay stealthed). Unfortunately you can only take one murderer with you at a time.