Ordeal of Purphoros is a uncommon Enchantment — Aura card from Pioneer Masters (PIO), collector number 149.
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.