Creature - Imp 1/1, 2B (3) Flying {T}: Each non-Wall creature the active player controls attacks this turn if able. At end of turn, destroy each of those creatures that didn't attack this turn. Play this ability only during an opponent's turn, before attackers are declared.1
Illus. Zina Saunders
It does affect creatures with summoning sickness. [D'Angelo 1998/07/30] So put your creatures into play after your attack.
If the creature cannot attack because of some restriction, such as Pacifism, or because it is tapped, it is destroyed at the end of the turn. [D'Angelo 1998/07/30]
If the Imp leaves play before the end of the turn, the creature still dies. [D'Angelo 1998/07/30]