All-Seeing Arbiter

Flying
Whenever this creature enters or attacks, draw two cards, then discard a card.
Whenever you discard a card, target creature an opponent controls gets -X/-0 until your next turn, where X is the number of different mana values among cards in your graveyard.

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Flying
Whenever All-Seeing Arbiter enters the battlefield or attacks, draw two cards, then discard a card.
Whenever you discard a card, target creature an opponent controls gets -X/-0 until your next turn, where X is the number of different mana values among cards in your graveyard.

The mana value of a double-faced card in your graveyard is always the mana value of the front face.

The mana value of a split card in the graveyard is the total mana value of both halves of that card. It does not have two mana values.

X is 0 when determining the mana value of a card in your graveyard.