Lightning Trap

When you have Lightning Trap in hand, reveal
it, then entomb it. ( To entomb a card, Exile it. For
as long as that card remains exiled, you may cast it
by tapping an untapped creature you control in
addition to paying any other costs. )
If you dealt combat damage this turn, you may
cast Lightning Trap without paying its mana cost.
Lightning Trap deal 4 damage to target creature.