Dark Brotherhood Quests
Dark Brotherhood quests mostly involve dungeon-crawling. There are no non-member quests.
Membership Quest: generating the invitation to join the DB works on a point system with points being awarded for the number of people you kill within a 24-hour period. 5 points are awarded for each "townie" (the people who wander around the towns) and 1 point is awarded for each town guard you kill. When you accumulate 15 points within 24 hours, the invitation will generate and you will receive it about 10 game days later.
The letter tells you to meet an NPC at an alchemist's shop in a town. When you show up and click on the contact, you will be given a vial of snake venom and instructions to put it in a decanter in a particular house. You must find the house and then find the wine decanter within the house. When you click on the decanter, it will be moved into your inventory and the poison will mix automatically. All you must do from there is "remove" it from your inventory and report to the designated contact in the proper town.
A couple of players have complained that they could not find the decanter, but I have never experienced this problem. Be sure that you do not let the time expire for this quest and that you are in the right house (some houses look like one house on the town map, but are actually two houses in the view window, and vice versa).
If you fail this quest, you get to fight an assassin or two when you report to your contact. If you succeed, you are admitted to the DB and get a gold piece for your troubles (the gold piece part doesn't work consistently).
"There is a traitor in the Brotherhood..." or "From time to time, the Brotherhood is forced to do an accounting in our own books..." (any rep): you are looking for a Nightblade (watch out for spells once you are 4th or 5th level), but it's a standard dungeon crawl. No fee for this one.
"...there is no room for failure in the Dark Brotherhood..." or "...It is a solemn duty, and not an entirely disagreeable one, to account agents of the Brotherhood who have failed us..." (any rep): go talk to the snitch to get the location of your target. A standard dungeon crawl. No fee for this one.
"There is another assassination group budding in <province>..." or "The Dark Brotherhood has been the sole assassin's guild in Tamriel since the Second Era..." (any rep): you are after a freelance assassin, but the DB would like for you to get the boss, too. Once you kill the freelance, you should find a note on his body with the boss' location (another dungeon). Go there and take out the boss. Both are standard dungeon crawls, but keep an eye on the calander. If you are pressed for time, the DB will settle for just the freelance. No fee for this one.
"The Mages Guild has come to us for a bit of messy business..." or "We have the Mages Guild for a client again..." (any rep): a standard dungeon crawl. The dialogue tells you that the target has magical stuff that you can keep, but this usually isn't the case. Be prepared for a spellcaster, but you'll get no fee.
"Here's a rather amusing accounting that may appeal to your morbid sense of humor..." (rep 10 or more): the target is a Bard with poor taste in song lyrics. A standard dungeon crawl.
"The Brotherhood prides itself on always knowing where each Brother is at all times..." or "As you know, no one ever leaves the Dark Brotherhood..." (rep 10 or more): go talk to the cousin. The cousin's clue should forewarn you that you're dealing with a vampire (if it doesn't, I'm telling you that you're dealing with a vampire). A standard dungeon crawl, otherwise.
"...a caddish knight has besmeared the honor of a fair damsel and refuses to fight any who would defend her..." (rep 20 or more): a standard dungeon crawl with a Knight as your target.
"...You are growing arrogant, egotistical, and vain. It's wonderful to see..." (rep 30 or more): you are to account a Mage (a Sorcerer, usually), but he's not in a dungeon. Instead he's at a Mages Guildhall in another town. Allegedly he has buddies, but all I've ever run into is the target. Of course, I've never taken this one while the Guildhall was actually open, so that may have something to do with it.
"We have an anonymous job for you..." (rep 30 or more): the target is in a town (usually in the Palace). Sometimes he has buddies. Success will usually boost your rep with the nobility in that province.
"This little excursion is not for just anyone..." (rep 40 or more): the target (a Knight) is in a Palace. Success will usually boost your rep with the nobility in that province.
"We have an interesting job available.... A high ranking member of the Thieves Guild..." (rep 50 or emore): this one is a little difficult because the target moves around quite a bit. Utilize the spymaster to get the skinny on where the target is and at what time. If you time it right, you can get the target without any buddies.
"This could be a tough one, even for you..." (rep 60 or more): your target is a warrior with a limp (actually a vampire). Can be a tough fight for low-level characters.