It is a one-mana instant spell that gives you one black mana for each creature card in your graveyard
If you cast eight spells, you get 255 tokens, which should be enough to end any game, unless something really weird happens
Green has loads of in-color synergies that work, and you don't even need to do this as a sorcery, so you can wait until the end of someone's turn, tap out and discard a card and dig up something nice and toss it onto the battlefield
Karvanista, Loyal Lupari Karvanista, Loyal Lupari works well in pretty much any human-themed Commander deck
It's been a major player in a variety of decks, getting banned in Modern due to just how oppressive it was in Rakdos Scam decks
A deck that has a lot of synergy but had a bad mana base and a bad mana curve will still be clunky and awkward