Blast Zone

This land enters with a charge counter on it.
{T}: Add {C}.
{X}{X}, {T}: Put X charge counters on this land.
{3}, {T}, Sacrifice this land: Destroy each nonland permanent with mana value equal to the number of charge counters on this land.

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Modern Horizons 3 Commander

Blast Zone enters the battlefield with a charge counter on it.
{T}: Add {C}.
{X}{X}, {T}: Put X charge counters on Blast Zone.
{3}, {T}, Sacrifice Blast Zone: Destroy each nonland permanent with mana value equal to the number of charge counters on Blast Zone.

Commander Masters

The Brothers' War

An activation cost of {X}{X} means that you pay twice X. If you want X to be 3, you pay {6} to activate Blast Zone's ability.

If a permanent has {X} in its mana cost, X is considered to be 0.

Tokens that aren't a copy of something else don't have a mana cost. Anything without a mana cost normally has a mana value of 0.