You can Ritual Summon this card with a "Libromancer" card. If this card was Ritual Summoned by using a monster(s) on the field, your opponent cannot target Ritual Monsters you control with monster effects. If this card is Special Summoned: You can target 1 face-up monster your opponent controls; until the end of this turn, change its ATK to 0, also negate its effects. You can only use this effect of "Libromancer Mystigirl" once per turn.