Atemsis, All-Seeing
Legendary Creature — Sphinx
Flying
, 
: Draw two cards, then discard a card.
Whenever Atemsis deals damage to an opponent, you may reveal your hand. If cards with at least six different mana values are revealed this way, that player loses the game.
Ryan Pancoast
Core Set 2020 Promos (PM20)
• #46pRulings
Every Magic card has exactly one mana value. A card with no mana cost has a mana value of 0, and 0 can be one of the six mana values that count toward Atemsis's ability.
(wotc • 2019-07-12)
The mana value of a split card is determined by combining its two halves. For example, the mana value of Fire//Ice in your hand is 4.
(wotc • 2019-07-12)
If a card in your hand has {X} in its mana cost, the value of X is considered to be 0.
(wotc • 2019-07-12)
If an effect causes you to play with your hand revealed, you can still reveal your hand as Atemsis's last ability resolves. The ability works as normal.
(wotc • 2019-07-12)
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Rulings
Every Magic card has exactly one mana value. A card with no mana cost has a mana value of 0, and 0 can be one of the six mana values that count toward Atemsis's ability.
(wotc • 2019-07-12)
The mana value of a split card is determined by combining its two halves. For example, the mana value of Fire//Ice in your hand is 4.
(wotc • 2019-07-12)
If a card in your hand has {X} in its mana cost, the value of X is considered to be 0.
(wotc • 2019-07-12)
If an effect causes you to play with your hand revealed, you can still reveal your hand as Atemsis's last ability resolves. The ability works as normal.
(wotc • 2019-07-12)