Grestos-Xal, Rune Guardian

Whenever Grestos-Xal enters the battlefield
, exile the top 20 cards of your library, you may look at them. Separate them into two piles, and then target player chooses a pile while its face down. Put every creature and land card from among that pile onto the battlefield, put the rest into exile, you can play them without paying their mana costs any time you could play an instant. And then shuffle the other pile into
your library.