Cloak of Confusion

Enchant creature you control
Whenever enchanted creature attacks and isn't blocked, you may have it assign no combat damage this turn. If you do, defending player discards a card at random.

Masters Edition II

Enchant creature you control
Whenever enchanted creature attacks and isn't blocked, you may have it deal no combat damage this turn. If you do, defending player discards a card at random.

Fifth Edition

o0: Defending player discards a card at random. Enchanted creature deals no combat damage this turn. Use this ability only if enchanted creature is attacking and unblocked and only once each turn.

Ice Age

If target creature you control attacks and is not blocked, you may choose to have it deal no damage to defending player this turn. If you do so, that player discards a card at random from his or her hand. Ignore this ability if that player has no cards in hand.

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.