Cheapest priced copy (Fifth Edition): $0.35 at Card Kingdom, near mint, their price list as of .
Errantry 
Enchantment — Aura · Common · Ice Age · #183
Enchant creature
Enchanted creature gets +3/+0 and can only attack alone.
Legal inCommanderLegacyVintagePauper
Not legal inBrawlStandardPioneerModern
2 printings · $0.35 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Fifth Edition | 224 | Common | $0.35 | — |
| Ice Age | 183 | Common | $0.35 | — |
2 rulings from Wizards of the Coast
- 1 October 2008“Can only attack alone” means the enchanted creature can’t be declared as an attacker during the declare attackers step unless no other creatures are declared as attackers at that time (either by you or your Two-Headed Giant teammate).
- 1 October 2008If the enchanted creature attacks alone, then a spell or ability causes another creature to be put onto the battlefield attacking, that’s fine.