Necropotence takes the simple equation of "one card = one life" and turns it on its head
A rare nobody plays can be bulk, while a common that sees play in Commander, Pauper, Legacy, or Modern can be worth more than a handful of rares
Something as simple as Viscera Seer works with a Blood Artist effect to blast your opponents to pieces as the assassins leap into combat then disappear without a trace
Smothering Tithe generates the most dominating mana advantage you can get without resorting to fast mana
It gives black a version of Propaganda or Ghostly Prison which would be very good
It requires some dedicated support to get it out early, thanks to its eight mana cost, but once its out there, the opponents will have a hard time dislodging it