Niambi, Esteemed Speaker — ruling 1

If a creature on the battlefield has {X} in its mana cost, X is considered to be 0.

Niambi, Esteemed Speaker — ruling 2

If the target creature is an illegal target by the time Niambi's triggered ability tries to resolve, the ability won't resolve. You won't return the card from the zone it moved to or gain any life.

Niambi, Esteemed Speaker : règles, fonctionnement et interactions (MTG)

Niambi, Esteemed Speaker 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

Flash When Niambi enters, you may return another target creature you control to its owner's hand. If you do, you gain life equal to that creature's mana value. , , Discard a legendary card: Draw two cards.

How it works : Niambi, Esteemed Speaker

Original English text — Wizards of the Coast / Scryfall

#1June 23, 2020· Wizards of the Coast
If a creature on the battlefield has in its mana cost, X is considered to be 0.
No other card is named in this ruling.
#2June 23, 2020· Wizards of the Coast
If the target creature is an illegal target by the time Niambi's triggered ability tries to resolve, the ability won't resolve. You won't return the card from the zone it moved to or gain any life.
No other card is named in this ruling.