If more than one card is exiled by Drach'Nyen, usually because its triggered ability was copied one or more times, X is the total power of the exiled cards.

If the exiled card's power is determined by a characteristic-defining ability, usually denoted by a "*" in the power/toughness box, that ability applies in exile and should be used to determine the value of X.

If the exiled card doesn't have a printed power, perhaps because it was a noncreature permanent that was only a creature on the battlefield due to another effect, X is 0.