Natural Unity is a common Conspiracy card from Conspiracy: Take the Crown (CN2), collector number 9. Current market price: $0.13 (foil $0.25), TCGplayer market prices via Scryfall.
Hidden agenda (Start the game with this conspiracy face down in the command zone and secretly choose a card name. You may turn this conspiracy face up any time and reveal that name.) Creatures you control with the chosen name have "At the beginning of combat on your turn, you may pay {G}. If you do, put a +1/+1 counter on this creature."