Now HERE is a quest that takes the entire Dark Brotherhood questline and puts it on its head. You get this quest by KILLING ASTID when you first meet her in the quest "With Friends Like These…". She’s not too hard to kill given the fact that you get several free hits in as she hops down off that bookcase, but once you do kill her this quest will be given and you’ll be UNABLE to complete any other Dark Brotherhood quest.
With the leader of the Dark Brotherhood dead, you can free the captives, loot her body, and escape. You’ll then be told to report what you did to any guard (and it really doesnt’ matter who you report it to). Once you do that, you will be told to report to Commander Maro, in the village of Dragon Bridge.
Head on over to Dragon Bridge and enter the Penitus Oculatus Outpost. Tell Commander Maro the news and he’ll be ecstatic. He’ll tell you the time to strike is now and give you the passcode for the Dark Brotherhood’s Sanctuary door: "Silence, my Brother". He’ll tell you to head over there and put everyone inside to the sword, as it should be "our honor". Honor? This guy wants ONE person to take on the whole place. What a load!
Well, regardless of the "Commander’s" poor decision-making skills, head on over to the Dark Brotherhood Sanctuary west of Falkreath and head inside the sanctuary. You will likely meet Arnbjorn where Astid usually hangs out, in their bedroom, which is fantastic as you can fight him one on one. Take him out (he doesn’t transform into a werewolf) and loot the upper level here. Be sure to take the "Unusual Gem" from the bedroom here. Also be sure to read the ALTERATION SKILLBOOK "Sithis" and grab all the enchanted armor.
Head further in now and down by the smithery is Veezara. His attacks can HURT plus he can heal himself, so pace yourself here and keep on the attack. Thankfully you can fight him one-on-one here. Loot his magic clothes after and head up the stairs on the right (to the right of the ones you came down). In the room on the left up here you should find the magician, Festus Krex. Rush him and take him down before he can start casting magic. Another one on one fight! Notice that the Night Mother is not here yet (meaning you can’t interact with her or kill Cicero).
After killing Festus loot him and go through the next door. In front of you search the room for the SNEAKING SKILLBOOK "Sacred Witness" and then head to the next hallway towards the marks. Head right and rush Nazir, as you are able to fight him one on one in this bedroom area above the dining room. He has no enchanted items, but the Hammerfell garb is pretty cool. One mark left, so head to the Alchemy area and take out Gabriella and her pet spider Lis. After taking her out, feel free to look around and loot the Sanctuary and be sure to find the DRAGON WALL for the KILL - MARKED FOR DEATH dragonshout word of power.
Once you’re done with the slaying, head back outside and fast travel back to Dragonbridge so you can report to Commander Maro. For your efforts of single-handidly wiping out the Dark Brotherhood (with the obvious exceptions of Babette and Cicero) you will be rewarded with 3,000 gold. Well, and all that stuff you picked up in the sanctuary (see why this path just isn’t worth it in the long run?). Commander Maro will even mention that the Emperor will hear about what you’ve done. Congrats!