Cheapest priced copy (Theros): $0.35 at Card Kingdom, near mint, their price list as of .
Ordeal of Erebos 
Enchantment — Aura · Uncommon · Theros · #99
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, target player discards two cards.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Theros | 99 | Uncommon | $0.35 | $0.49 |
3 rulings from Wizards of the Coast
- 15 September 2013The +1/+1 counter is put on the creature before blockers are declared and before combat damage is dealt.
- 15 September 2013The check of whether the enchanted creature has three or more +1/+1 counters on it happens as part of the resolution of the attack triggered ability. If the third +1/+1 counter is put on the enchanted creature any other way, you won't sacrifice the Ordeal until the next time the creature attacks.
- 15 September 2013If you sacrifice the Ordeal in some other way, its last ability will trigger.