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.

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.

If a card in your hand has in its mana cost, the value of X is considered to be 0.

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.