Grimoire of Myths

Empty {g} (You may cast this card for its empty cost. If you do, exile it as it resolves. During your precombat main phase, if an opponent cast no spells last turn, you may cast the exiled card without paying its mana cost. Empty only as a sorcery.)
When Grimoire of Myths enters the battlefield, target opponent shuffles his or her library.
{t}: Add {1} to your mana pool.