You choose the target creature or planeswalker as the triggered ability of Wolf of Devil’s Breach is put onto the stack. You choose which card to discard, if any, as the ability resolves. While players may respond to the triggered ability once you’ve chosen a target, no player may take actions between the time you discard the card and the time damage is dealt.

If a card with {X} in its mana cost is discarded to Wolf of Devil’s Breach’s ability, X is considered to be 0.

You can’t choose to discard a card without also paying {1}{R}.