X (value)

From Archon Arcana

Sometimes X is used to specify a value that is defined by a card ability. Unless defined by a card ability, the value of X is equal to 0.

Example: Picaroon has a power of X and the ability "X is the combined power of Picaroon's neighbors." If your opponent plays Shadow of Dis, Picaroon will no longer have that ability, so Picaroon's power will be 0.

Official rules v1.7 December 2020