Dark Brotherhood

Membership in the Dark Brotherhood is by invitation only.  The invitation to join will be issued when you kill three townspeople (or a lot of guards) within a 3-day period. Complete their quest to gain membership.  The guild makes not-so-veiled threats about what will happen to you if you don't join, but nothing has ever materialized for me aside from a severely negative rep (-25 points) with the DB.  The quest can be a bit buggy with the letter containing your instructions appearing and then suddenly disappearing.  If you get the letter, save the game immediately so you'll have something to go back to.

Required Skills:

• Archery, Backstabbing, Climbing, Critical Strike, Daedric, Destruction, Short Blade, Stealth, Streetwise

Training Offered:

• Guild skills plus Dodging, Running, Swimming

Benefits of Membership:

• Guild operations are 24/7 (they don't even close for Christmas)
• Chance of bribing the judge when you’re charged with murder (chance increases with your rank in the guild)
• A new graveyard revealed on your map at each rank
• Access to Potionmaker at rank 3 (lower than any other guild)
• Buy Soul Gems at rank 5 (have to join Julianos and/or Mages Guild to use them, though)
• Access to Spymaster at rank 7

Dark Brotherhood quests all seem to involve finding some bozo at the bottom of a dungeon and offing him. The pay is so-so, non-existant in some cases, and the access to the potion maker at rank 3 and Spymaster at rank 7 are the only real benefit to joining unless you get your jollies out of dungeon-crawling (which can be a very profitable enterprise) and bopping townspeople (who die if you sneeze at them).

A final comment: at the time of your promotion to rank 5, it is stated that you can sell soulgems.  This is not true.  I have not been able to find anyplace in the game where you can sell a soulgem, most especially at the Dark Brotherhood.