Exodus classical source of black mana. ^^
Slightly different than "Songs of the Damned" from Ice age, that gives black mana for the humber of creature in your graveyard, "Culling the weak" gives you some black mana when you sack one of your creature.
If yoiu need a bunch of black mana the combination of the 2 cards can be graceful.
Anyway even alone "Culling the weak" can works a bunch with some specific decks.
I let you imagine wich one of course ;)