Nemesis of Mortals is a uncommon Creature — Snake card from Theros (THS), collector number 163. Current market price: $0.08 (foil $0.40), TCGplayer market prices via Scryfall.
This spell costs {1} less to cast for each creature card in your graveyard. {7}{G}{G}: Monstrosity 5. This ability costs {1} less to activate for each creature card in your graveyard. (If this creature isn't monstrous, put five +1/+1 counters on it and it becomes monstrous.)