Oath of Nereleb

When Oath of Nereleb enters the battlefield, reconsider 2.
Whenever you cast a planeswalker spell, target opponent exiles the top card of his or her library. Until end of turn, you may cast that card without paying its mana cost if it's an artifact, enchantment, instant, or sorcery.
"In the stead of my negligent father, I will reluctantly keep watch."