Cheapest priced copy (Legends): $17.99 at Card Kingdom, near mint, their price list as of .
Infinite Authority 
Enchantment — Aura · Rare · Legends · #22
Enchant creature
Whenever enchanted creature blocks or becomes blocked by a creature with toughness 3 or less, destroy the other creature at end of combat. At the beginning of the next end step, if that creature was destroyed this way, put a +1/+1 counter on the first creature.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $17.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Legends | 22 | Rare | $17.99 | — |
2 rulings from Wizards of the Coast
- 4 October 2004The enchanted creature gets a +1/+1 counter for each creature that triggers the ability.
- 15 October 2006Moving the enchantment after the ability triggers will not affect which creatures are affected.