Bark of Dark Ruler (TU04-EN010) [Turbo Pack: Booster Four]

You can only activate this card during the Damage Step of a Battle Phase if a Fiend-Type monster on your side of the field battles. Pay Life Points (in multiples of 100 points) to decrease 1 of your opponent's monster's ATK and DEF by the same amount of points until the End Phase of that turn.