2019-07-12 : 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.

2019-07-12 : If a card in your hand has {X} in its mana cost, the value of X is considered to be 0.

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.

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.