Skyclave Apparition
Creature — Kor Spirit
When Skyclave Apparition enters the battlefield, exile up to one target nonland, nontoken permanent you don't control with mana value 4 or less.
When Skyclave Apparition leaves the battlefield, the exiled card's owner creates an X/X blue Illusion creature token, where X is the mana value of the exiled card.
Illustrated by
Game Day Promos#2RareEnglishLegality
Rulings
If a creature on the battlefield has 
in its mana cost, X is considered to be 0.
If Skyclave Apparition leaves the battlefield before its first ability resolves, the ability still exiles the target permanent.
If there’s no exiled card when Skyclave Apparition leaves the battlefield (most likely because its first ability hasn’t resolved yet), no player creates a token.
If Skyclave Apparition’s first ability exiled more than one card owned by a single player, that player creates a token with power and toughness equal to the sum of those cards’ converted mana costs. If the first ability exiled cards owned by more than one player, each of those players creates a token with power and toughness equal to the sum of the converted mana costs of all cards exiled by the first ability.