Coalition Flag is a uncommon Enchantment — Aura card from Apocalypse (APC), collector number 2. Current market price: $0.25 (foil $2.32), TCGplayer market prices via Scryfall.
Enchant creature you control Enchanted creature is a Flagbearer. While an opponent is choosing targets as part of casting a spell they control or activating an ability they control, that player must choose at least one Flagbearer on the battlefield if able.