Arbinaq's Entropy

You attempt to suppress and erode the magical capabilities of a creature. Choose a creature you can see within range. The target must succeed on an Intelligence, Wisdom, or Charisma saving throw based on its spellcasting ability modifier. On a failed save, the target loses 1d4 spell slots beginning with lower level spell slots and working up and is unable to cast one spell that you have seen it cast that is of half this spells level (rounded up).

At the end of each of the creatures turns, it can make another saving throw against this spells effect to regain its lost spell. If the target fails its save three times in a row, the ability to cast that spell is lost from its mind and it is unable to cast it again until targeted by a greater restoration spell or similar magic.

At Higher Levels. When you cast this spell using a spell slot of 5th, 6th, 7th, 8th, or 9th level, the die size of spell slots the target loses increases to a d6, d8, d10, d12, and d20 respectively.