Gríma, Saruman's Footman

If the spell you cast has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you cast a spell "without paying its mana cost," you can't pay any alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.

If that player exiles their entire library without exiling an instant or sorcery card, they will randomize the order of the exiled cards, and the cards then become that player's library, ending the effect. They will not continue to exile and randomize their library forever.