Underdark Beholder

If Underdark Beholder leaves the battlefield while its last ability is on the stack, use the number of eyestalk counters it had before it left to determine what converted mana cost to look for.

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

Underdark Beholder’s second ability isn’t a damage prevention effect. It can replace damage that can’t be prevented.

You must cast the revealed card while Underdark Beholder’s triggered ability is resolving. You can’t cast it later in the turn.