Oracle Text

Enchant creature

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

Card Rulings

10/1/2008 “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).
10/1/2008 If the enchanted creature attacks alone, then a spell or ability causes another creature to be put onto the battlefield attacking, that’s fine.