It's derived from the traditional suicide black in function, but this should, hopefully, be more powerful. Yawgmoth's Bargain speeds up your hand faster than Necropotence, but it has a higher casting cost. Hero's Downfall, Big Game Hunter, and Avatar of Woe should keep your opponent's creatures down, and Exsanguinate will be your killing card. Lake of the Dead and Dark Ritual give massive mana ramp, even if it only lasts one turn. This should be used on Exsanguinate or Yawgmoth's Bargain most of the time, but some special cases will arise. Crypt Ghast will give even more mana ramp, especially when you have Urborg, Tomb of Yawgmoth, as then, when you use Lake of the Dead's secondary ability, you get an extra black mana from it as well.
Vampire Nighthawk
will supplement your lifegain, and thus your card drawing, and Vampire Nighthawk will make your opponents hesitate to attack you. Demonic Tutor is a useful card in any situation, and Venser's Journal means you are not only not discarding, but regaining all the life you spent on drawing.
On the sideboard,
Kagemaro, First to Suffer
is a really powerful card that, in addition to being able to clear the field, can be used as a massive creature. He's mostly for dealing with token creature decks and indestructible decks, but he might work in other situations too. Crypt Incursion and
Suffer the Past
are for those pesky graveyard dig players, and really don't have much use beyond that.
Dark Betrayal
is really strong against mono-black or dual colored decks that use black, but in most cases Hero's Downfall will work better, just more expensive.
Update 1:Removed Whip of Erebos, Hymn to Tourach and 2 Elixir of Immortality; added 2 Diabolic Tutor, 3 Mind Twist and 2
Extirpate
Update 2:Removed 2 Crypt Incursions from sideboard to replace with Erebos, God of the Dead, for pesky lifegain decks.
Update 3: Replaced 4 swamps with Cabal Coffers for extra mana.
Update 4: Replaced 2 Venser's Journal for 2 Nevinyrral's Disk. Should help save this deck when close to defeat.