Fire Lord Sozin's triggered ability can target any combination of creature cards whose mana values total X or less. Regardless of what other creature cards you choose as targets, you can also choose any number of creature cards with mana value 0 as targets.

You don't have to choose the name of a card that you've already seen this game; you may guess at a card you think your opponent might have in their hand or library if you'd like.

You target an opponent when The Rise of Sozin's second chapter ability is put on the stack, but you don't name a card until it resolves. Notably, this means that players don't get to respond to the ability knowing what card you will name.

This ability has no effect on permanents with the chosen name that are on the battlefield.

Even if you choose a card name (for example, a basic land) that appears more than four times in your opponent's deck, you may only choose up to four copies of the card. You can also always choose fewer than four copies of the card to exile, even if there are more in the deck.