Dark Brotherhood
Overview
When someone needs to arrange a hit they pray to the Night Mother. The Night Mother then sends her children to do the job. In a sick and twisted cult the Dark brotherhood is a combination of a dangerous cult and a merderous mafia. Either way they're assassins of the highest degree.
Joining
You don't find the Dark Brotherhood, the Dark Brotherhood finds you. If you commit a murder without witnesses, the Dark Brotherhood will come calling. Note that the victim must be innocent and killed in cold blood – not someone who has first attacked you. When you have killed someone appropriate you will receive the message “Your killing has been observed by forces unknown”.
After committing your foul deed, you must sleep in a secure location, preferably outdoors (any bandit camp will do, as will any tents outside forts or Ayleid Ruins). Lucien Lachance will interrupt your sleep and task you with another murder, as well as give you a Blade of Woe (a pretty unremarkable blade, outside of its unique design). Accomplish this, and you will be a member of the Dark Brotherhood.
Tenets and Explusion
Even as a cult full of cold blooded killers the Dark Brotherhood has a set of tenets. Oddly enough though it's impossible to break the first three to my knowledge.
Tenet 1: Never dishonor the Night Mother. To do so is to invoke the Wrath of Sithis.
Tenet 2: Never betray the Dark Brotherhood or its secrets. To do so is to invoke the Wrath of Sithis.
Tenet 3: Never disobey or refuse to carry out an order from a Dark Brotherhood superior. To do so is to invoke the Wrath of Sithis.
Tenet 4: Never steal the possessions of a Dark Brother or Dark Sister. To do so is to invoke the Wrath of Sithis.
Tenet 5: Never kill a Dark Brother or Dark Sister. To do so is to invoke the Wrath of Sithis.
If you get thrown out of the guild for breaking into those then you still have a chance to get back in. Next time you sleep you will be attacked by "The Wratch of Sithis" who is a great big ghost with a knife. And let me tell you he is tough. Weapons don't work so if you're not a mage you're pretty screwed.
The first and second time you're expelled killing the ghost is enough to get back in. The third time, however, there is no forgiveness even if you do kill the ghost.
Most of the Dark Brotherhood quests will consist of assasinations, often with conditions attached. If you fulfil these conditions then you will get a bonus usually a lot more money or a rare item, it's almost always worth doing it right.
Vincente Valtieri Quests
A Watery Grave
Ok this time you've got to kill a pirate onboard his ship. Theres no bonus so just go at night when he's sleeping. Go to the Waterfront district of the Imperial city and there should be 2 ships, The floating Bloat and the pirate ship. Walk onboard the pirate ship and go into sneak mode and go into the small extremely obvious door to your left. The captain should be sleeping in his bed a few feet away. Stay in stealth mode and stab him in the back a few times once he's dead loot him quickly and get off the boat. Pirates will come to see whats going on and if they see you you'll have an uneccesary fight on your hands.
Accidents Happen
This quest is the first time you have a bonus. A man needs to be killed but it has to look like an accident. That rules out the usual stab 20 times in the face as well as bashing his face in with a mace. So what to do? When you're getting the quest Valtieri suggests you loosen the Minotaur head above the victims chair so we'll try that if we want the bonus.
Before you do this though make sure you have a few lockpicks. If you need you can buy em from Ongar a few buildings away from your victim. That is if you're in the Thieves' Guild of course. The best time to go into the house is 8 pm since it gives you plenty of time to get it done.
So go around behind his house and pick the lock to break into his basement. You're going to want to be in stealth mode from here on out. You can steal whatever you want down here since there is no one downstairs. Now pick the lock on the door up to the first floor, and remember BE IN STEALTH MODE. No sneak along in the shadows. If you look to your right you'll see your target sitting contently talking with his huge orc body guard, good thing we don't have to fight him. So carefully sneaking around the doorway and head upstairs. You can steal whatever you want from up here too since no one is looking. In one of the two bedrooms there will be a portion of wall that you can open and enter a small passage way. Move to then end and make sure it's between 8 and 11 (Thats when he's in the chair) and undo the knotting and listen as it falls and kills him.
Now you'd think the orc would be after you, but since he thinks it's an accident he just stands gasping at his master. DO NOT KILL THE ORC, if you do then you will not get the bonus for making the quest look like an accident. Steal anything you still want and sneak out the way you came. Head back to the Dark Brotherhood to turn in. Valtieri will give you your money as well as Sufferthorn (a fairly nice dagger) if you made it look like and didn't kill the orc like I told you to.
Scheduled for Execution
You know you've been thinking about this since the first minutes of the game. Killing that damn insulting Dark Elf in the cell across from you. When that secret passage opened up you thought to yourself "someday when I'm powerful I will come back through here and kill him." Well now you get to do it and you get paid to boot.
Vincent gives you the key that lets you get back into the sewers you escaped from so very long ago. Head back in and you'll find it's full of guards. Really they probably only needed one or two guards stationed at the entrance, but whatever.
So sneak through the sanctuary and come back to your old cell. Wait for a moment and listen to the Dark Elves' crazy dreams of escape and glory. Wait for the guard to leave, open your door and shoot a couple arrows through the bars at him.
Head up the stairs and walk out of the prison, head back to Vincent for your reward. Still can't believe you got paid to do that can you?
The Assassinated Man
Now this quest is a bit on the odd side. A man has hired assassins to fake his death. Yeah thats a good idea. But we get a bonus so off we go.
When you're assigned the quest Vincent gives you a poison dagger and the antidote (you see the plan yet?) Meet the "victim" (Motierre) and talk to him, he'll ask you to wait for the real assassin (gangster) to show up. Quickly stab Motierre with the poison dagger. The other assassin will try to attack you (Why? you just did his job for him) so run out of the house fast.
Come back in town and sleep/wait for about a day. Head to the church basement and give him the antidote. He'll mention that this is his family's tomb. Then he'll mention that his family will see this as desecration and LOOK A ZOMBIE. So you'll have to fight 3 or 4 zombies down here (what do ya know you got to kill something after all!) and escort him to the inn (he apperently thinks that a public place would be a good place to hide) and head back to Vincent for your reward.
Ocheeva Quests
The Lonely Wanderer
Bad Medicine
Whodunit?
This is one of the most fun you'll have on a quest. Five people all targets are locked in a house (willingly) and told to search for a treasure chest with gold and the key to the house. The catch? There is no treasure and the key is with the assassin sent to kill them all. You.
Heres where it gets fun, if you can do it without being seen then you get a bonus.
The first target will be the guy who goes upstairs to take a nap, don't stab him or the dark elf woman will walk in on you. Sneak into the corner that can't be seen from the door and shoot him with an arrow. For fun talk to everyone and watch their reactions.
Next target should be the woman who comes upstairs, no one will come upstairs so be as loud as you want.
Third should be the rich elf. He hangs out downstairs and it's easy to get a sneak attack on him that no one sees.
With three people dead the old woman goes paranoid and the nord will head down to the basement to get himself nice and drunk. Follow him down and kill him by any means you see fit.
Lastly is the old woman. Speak with her and she won't even suspect you in your black clothes. Kill her as you see fit there's no one left to see you.
NOTE: This is far from the only way to do this quest. Be creative and do whatever you want. Some people say you can use speechcraft to make the whole house go to hell without you getting your hands dirty.