Black Skull Dragon, the Archfiend of Unity

Cannot be Normal Summoned/Set. Must be Special Summoned by its own effect. If this card is in your hand (Quick Effect): You can discard 1 Quick-Play or Ritual Spell; Special Summon this card in Defense Position. You can only use this effect of "Black Skull Dragon, the Archfiend of Unity" once per turn. If this card is Special Summoned: You can Set 1 Spell/Trap that mentions "Light and Darkness Ritual" from your Deck or GY. At the start of the Battle Phase: You can make your opponent lose 800 LP, and if you do, this card gains 800 ATK.