Ordeal of Purphoros is a uncommon Enchantment — Aura card from Theros (THS), collector number 131. Current market price: $0.16 (foil $0.20), TCGplayer market prices via Scryfall.
Enchant creature Whenever enchanted creature attacks, put a +1/+1 counter on it. Then if it has three or more +1/+1 counters on it, sacrifice this Aura. When you sacrifice this Aura, it deals 3 damage to any target.