Ordeal of Thassa - Theros - 58

Enchantment — Aura

Mana Cost: {1}{U}

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, draw two cards.

Uncommon · Theros · #58 (THS)

Illustrated by Howard Lyon