Frontier Siege is a rare Enchantment card from Fate Reforged (FRF), collector number 131. Current market price: $0.38, TCGplayer market prices via Scryfall.
As this enchantment enters, choose Khans or Dragons. • Khans — At the beginning of each of your main phases, add {G}{G}. • Dragons — Whenever a creature you control with flying enters, you may have it fight target creature you don't control.