Ordeal of Purphoros 
Enchantment — Aura
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.
Legality
standardNOT LEGALpioneerLEGALmodernLEGALlegacyLEGALvintageLEGALcommanderLEGAL