If a Fiend-Type monster you control battles, during the Damage Step: Pay LP (in multiples of 100 points), then target the opponent's battling monster; that opponent's monster loses that much ATK and DEF, until the end of this turn.
Rulings
Individual Card FAQs
This card targets your opponent's monster that is battling your Fiend-Type monster.
Paying Life Points for this card is a cost.
You cannot pay more Life Points than necessary. This means that if the target monster has ATK 1500 / DEF 1800, the highest amount of Life Points you can pay is 1800. The target monster will then be an ATK 0 / DEF 0. If the target monster has ATK 1850 / DEF 500, the most you can pay is 1800. This is because you have to play in multiples of 100, and paying 1900 would be paying more than necessary. So you may pay 1800 Life Points and the target monster would become an ATK 50 / DEF 0.
Netrep Rulings
"Bark of Dark Ruler" is a targeting effect.
Payment of Life Points is a cost to activate "Bark of Dark Ruler."
"Bark of Dark Ruler" may be activated in the Damage Step before Damage Calculation.
Trap|
Normal