Nightmarish End — ruling 1

The value of X is calculated as the ability resolves. The effect won't change later in the turn, even if the number of cards in your hand does.

Nightmarish End : règles, fonctionnement et interactions (MTG)

Nightmarish End 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

Target creature gets -X/-X until end of turn, where X is the number of cards in your hand.

How it works : Nightmarish End

Original English text — Wizards of the Coast / Scryfall

#1April 26, 2014· Wizards of the Coast
The value of X is calculated as the ability resolves. The effect won't change later in the turn, even if the number of cards in your hand does.
No other card is named in this ruling.