Aetherhold

When any opponent casts a spell, you may
discard a card. If you do, that spell enters the
battlefield as an X/X Reflection creature, where
X is its converted mana cost, loses all other
types and abilities and gains "When this
creature deals combat damage to any opponent,
it regains all its original abilities and types and
you may cast it without paying its mana cost.
If you don't, exile it."