Armament of Nyx is a common Enchantment — Aura card from Journey into Nyx (JOU), collector number 4. Current market price: $0.03 (foil $0.26), TCGplayer market prices via Scryfall.
Enchant creature Enchanted creature has double strike as long as it's an enchantment. Otherwise, prevent all damage that would be dealt by enchanted creature. (A creature with double strike deals both first-strike and regular combat damage.)