Arietra, First to Perish

When Arietra, First to Perish enters the battlefield, exile your hand. If you do, draw
cards equal to the amount of cards exiled this
way. You may repeat this process as many
times as you wish.
{x}{b}{r}{t}: You may cast a spell exiled with
Arietra with converted mana cost X without
paying its mana costs.