Dynamo, Professor

{3}{w}{w}{t}, if it is your turn, all enemy creatures must be declared as blockers during the combat phase this turn. As many of your creatures must be blocked as possible. If it is an opponent's turn, all enemy creatures must attack. All combat damage that would be dealt by opposing creatures is prevented this turn. Double the combat damage dealt by your creatures to the opponent's creatures this combat phase. Dynamo loses this ability.