Nemesis of Mortals is a common Creature — Snake card from Pioneer Masters (PIO), collector number 185.
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.)