Oracle Text

T: Untap target blue creature.

T: Choose target non-Wall creature the active player has controlled continuously since the beginning of the turn. That creature attacks this turn if able. Destroy it at the beginning of the next end step if it didn't attack this turn. Activate this ability only before attackers are declared.

Card Rulings

10/4/2004 The creature is destroyed if it does not attack because it simply can’t do so legally.
9/20/2013 If a turn has multiple combat phases, the ability can only be activated before the beginning of the declare attackers step of the first combat phase in that turn.