Melee

Instant    (CMC:5)

Cast only on your turn during combat before defense is chosen. Choose how attacking creatures you control are blocked; all defense must be legal. After declaring blocking, untap any unblocked attacking creature. Treat those creatures as though they had not attacked.

IA • ENDameon Willich

™ & © 1993—1995 Wizards of the Coast

Legal in: Ice Age Block,Legacy,Vintage,Commander

Oracle Text (click to copy):

4/15/2013
An ability that triggers when something "attacks and isn't blocked" triggers in the declare blockers step after blockers are declared if (1) that creature is attacking and (2) no creatures are declared to block it. It will trigger even if that creature was put onto the battlefield attacking rather than having been declared as an attacker in the declare attackers step.
9/20/2013
If a turn has multiple combat phases, this spell can be cast during any of them as long as it's before the beginning of that phase's Declare Blockers Step.