Thunderous Might is a uncommon Enchantment — Aura card from Born of the Gods (BNG), collector number 114. Current market price: $0.25 (foil $0.52), TCGplayer market prices via Scryfall.
Enchant creature Whenever enchanted creature attacks, it gets +X/+0 until end of turn, where X is your devotion to red. (Each {R} in the mana costs of permanents you control counts toward your devotion to red.)