Valefar Assimilator X

Menace
When Valefar Assimilator X deals combat damage to an opponent, look at the top 3 cards of that opponent's library. Exile one facedown. You may look at and cast that card, and you may spend mana as if it were mana of any type to cast that spell. Then; put a flying, lifelink, trample, deathtouch, first strike, reach, or vigilance, or +1/+1 counter on this card.