Steel Hellkite

The mana value of a permanent is determined solely by the mana symbols printed in its upper right corner, unless it's copying something else (see below). The mana value is the total amount of mana in that cost, regardless of color. For example, a card with mana cost {3}{U}{U} has mana value 5.

If a nonland permanent has no mana symbols in its upper right corner (because it's a token that's not copying something else, for example), its mana value is 0.

Steel Hellkite's last ability destroys only nonland permanents whose mana value is exactly equal to X, and only those controlled by players who have been dealt combat damage by Steel Hellkite this turn.