Drach'Nyen — ruling 1

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.

Drach'Nyen — ruling 2

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.

Drach'Nyen — ruling 3

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.

Drach'Nyen : règles, fonctionnement et interactions (MTG)

Drach'Nyen is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Echo of the First Murder — When Drach'Nyen enters, exile up to one target creature. Daemon Sword — Equipped creature has menace and gets +X/+0, where X is the exiled card's power. Equip (: Attach to target creature you control. Equip only as a sorcery.)

How it works : Drach'Nyen

Original English text — Wizards of the Coast / Scryfall

#1October 7, 2022· Wizards of the Coast
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.
No other card is named in this ruling.
#2October 7, 2022· Wizards of the Coast
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.
No other card is named in this ruling.
#3October 7, 2022· Wizards of the Coast
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.
No other card is named in this ruling.