Beastmaster's Magemark is a common Enchantment — Aura card from Guildpact (GPT), collector number 80. Current market price: $0.12 (foil $0.66), TCGplayer market prices via Scryfall.
Enchant creature Creatures you control that are enchanted get +1/+1. Whenever a creature you control that's enchanted becomes blocked, it gets +1/+1 until end of turn for each creature blocking it.