Necropanther
Creature — Cat Nightmare
Mutate 
(If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Whenever this creature mutates, return target creature card with mana value 3 or less from your graveyard to the battlefield.
Kev Walker
Magic Online Promos (PRM)
• #80955Rulings
If a card in a player’s graveyard has {X} in its mana cost, X is considered to be 0.
(wotc • 2020-04-17)
Rulings
If a card in a player’s graveyard has {X} in its mana cost, X is considered to be 0.
(wotc • 2020-04-17)