in its mana cost, X is considered to be 0.Skyclave Apparition removes the original card permanently; the opponent gets an Illusion later rather than getting that card back. That distinction makes the white creature a useful answer to small engines whose abilities are much more dangerous than their combat stats. Its entry ability can exile a nonland, nontoken permanent you do not control with mana value four or less.
Rhystic Study, a mana rock, or a troublesome planeswalker can all be suitable targets. When the Apparition leaves, the exiled card's owner creates an Illusion sized by that card's mana value. Giving an opponent a plain creature is often a favorable exchange for removing a permanent that repeatedly draws cards or controls the game. Tokens are not eligible targets, even when they would otherwise fall within the size restriction.
Blink decks can reuse the removal ability with Ephemerate or Thassa, Deep-Dwelling. Each departure can provide an Illusion, so repeated use trades several original threats for a growing set of ordinary bodies rather than clearing everything without compensation. That trade is strongest when your deck can handle combat but struggles with the engines being exiled. The double-white cost asks for dependable white sources early, and hexproof or other targeting restrictions still apply. Choose targets by the value of their abilities, not simply by selecting the largest legal permanent.
