Whenever Wolf of Devil's Breach attacks, you may payand discard a card. If you do, Wolf of Devil's Breach deals damage to target creature or planeswalker equal to the discarded card's converted mana cost.
4/8/2016 You can’t choose to discard a card without also paying .
4/8/2016 If a card with in its mana cost is discarded to Wolf of Devil’s Breach’s ability, X is considered to be 0.
4/8/2016 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.