Trapped Treasure

Imprint — Whenever Trapped Treasure enters
the battlefield or you cast the card exiled with it,
you may exile an instant card from your hand.
Whenever an opponent attacks you and/or one
or more planeswalkers you control, you may pay
{x}, where X is the mana value of the exiled card.
If you do, cast the exiled card without paying its
mana cost.
Unspeakable glory, unknowable danger...