Nature Shields Its Own is a common Ongoing Scheme card from Archenemy Schemes (OARC), collector number 32★. Current market price: $3.24, TCGplayer market prices via Scryfall.
(An ongoing scheme remains face up until it's abandoned.) Whenever a creature attacks and isn't blocked, if you're the defending player, create a 0/1 green Plant creature token that's blocking that creature. When four or more creatures attack you, abandon this scheme at end of combat.