Mists of Lórien — ruling 1

If a permanent has {X} in its mana cost, X is 0 when determining its mana value.

Mists of Lórien — ruling 2

If the target nonland permanent is an illegal target as Mists of Lórien tries to resolve, it won't resolve and none of its effects will happen. No permanents will be returned to their owners' hands.

Mists of Lórien — ruling 3

A copy of a spell can be countered like any other spell, but it must be countered individually. Countering a spell with replicate won't affect the copies.

Mists of Lórien : règles, fonctionnement et interactions (MTG)

Mists of Lórien 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

Replicate (When you cast this spell, copy it for each time you paid its replicate cost. You may choose new targets for the copies.) Return target nonland permanent and each other nonland permanent with the same mana value as that permanent to their owners' hands.

How it works : Mists of Lórien

Original English text — Wizards of the Coast / Scryfall

#1November 3, 2023· Wizards of the Coast
If a permanent has in its mana cost, X is 0 when determining its mana value.
No other card is named in this ruling.
#2November 3, 2023· Wizards of the Coast
If the target nonland permanent is an illegal target as Mists of Lórien tries to resolve, it won't resolve and none of its effects will happen. No permanents will be returned to their owners' hands.
No other card is named in this ruling.
#3November 3, 2023· Wizards of the Coast
A copy of a spell can be countered like any other spell, but it must be countered individually. Countering a spell with replicate won't affect the copies.
No other card is named in this ruling.