Mortal Obstinacy is a common Enchantment — Aura card from Journey into Nyx (JOU), collector number 17. Current market price: $0.23 (foil $0.26), TCGplayer market prices via Scryfall.
Enchant creature you control Enchanted creature gets +1/+1. Whenever enchanted creature deals combat damage to a player, you may sacrifice this Aura. If you do, destroy target enchantment.