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When the wielder is sent out in battle, it will take on the appearance of the last non-fainted Pokemon in the player's party. The Ball used, species name, shininess, nickname, cry, and gender are replicated, but typing, ability, stats, level, status, and moveset all remain the same. Illusion copies the forme of the last Pokemon as it exists in the party. This means that it will copy permanent formes, such as Deoxys formes, but will not copy formes that revert when a Pokemon is switched out, like Darmanitan's Zen Mode or Castform's Forecast formes.
Illusion fades if the wielder takes direct damage from an attack or loses Illusion in any way. Status conditions, entry hazards, weather, Substitute, and Life Orb will not cause the Illusion to fade. If the last Pokemon is a Pokemon with Illusion, then the wielder will take on the appearance of the species of the Pokemon in the final slot. If the wielder is the last Pokemon in the player's party and is switched in, Illusion will fail. The ability Imposter will fail if sent against a Pokemon with Illusion, regardless of whether the Illusion is broken or not. If the wielder uses Entrainment or Skill Swap, or is the target of Role Play or Skill Swap, the move will fail.
Illusion is an immensely powerful ability, and on something as volatile as Zoroark, it can be used to play extreme mind games with the opponent. What is notable is that Zoroark has a movepool very similar to most Ghost-types, and whereas Zoroark is weak to Fighting- and Bug-type moves, Ghost-types are immune or resistant, respectively. This makes the opponent wary of attacking, and as a result allows Zoroark to stay in play for longer. It's helpful in that it can either allow Zoroark to start an early sweep unhindered, or lets you scout for a counter to whatever you have Zoroark copying.
Found in Dream World