Yennett, Sovrana Criptica

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

Because all attackers are chosen at once, a spell cast this way can't affect which creatures you can attack with, but it can affect which creatures will be able to block.

If you wish to cast the revealed card without paying its mana cost, you must cast it while Yennett's triggered ability is resolving; you can't cast for free it later in the turn.

If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs, such as evoke costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Tormenting Voice, those must be paid to cast it.

If the revealed card doesn't have an odd mana value or if that card does but you choose not to cast it, you draw a card. Keep in mind that revealing a card doesn't cause it to change zones. This means that the card you draw will be the card you revealed.