"There is no shame in solitude. The lone knight may succeed where a hundred founder."
—Arna Kennerüd, skyknight

Errantry

Enchant creature
Enchanted creature gets +3/+0 and can only attack alone.

Masters Edition II

Fifth Edition

Enchanted creature gets +3/+0 and cannot attack during any turn in which any other creatures attack.

"There is no shame in solitude. The lone Knight may succeed where a hundred founder."
—Arna Kennerüd, Skyknight

Ice Age

Target creature gets +3/+0. If that creature attacks, no other creatures can attack this turn.

If the enchanted creature attacks alone, then a spell or ability causes another creature to be put onto the battlefield attacking, that’s fine.

“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).