Niambi, Esteemed Speaker
Legendary Creature — Human Cleric
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.
Eric Deschamps
Core Set 2021 (M21)
• #222Rulings
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.
(wotc • 2020-06-23)
If a creature on the battlefield has {X} in its mana cost, X is considered to be 0.
(wotc • 2020-06-23)
Rulings
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.
(wotc • 2020-06-23)
If a creature on the battlefield has {X} in its mana cost, X is considered to be 0.
(wotc • 2020-06-23)