Illusion

Illusion causes the Pokemon to enter battle disguised as the last Pokemon in the party that is still able to fight. The disguise copies the species appearance, nickname, Poke Ball, and even the Shiny status of the mimicked Pokemon, making it visually indistinguishable from the real thing to the opponent. This creates mind games — the opponent cannot be sure whether they are facing the disguised Pokemon or the real one until the illusion is broken.

The illusion breaks when the Pokemon is hit by any damaging move. Importantly, indirect damage from weather, Spikes, Stealth Rock, Leech Seed, or status conditions does NOT break the illusion, nor does damage dealt to a Substitute. This means a skilled player can maintain the disguise even while taking residual damage. If the battle's party order gets shuffled as Pokemon are switched in and out, Illusion always uses the last battle-capable Pokemon in the current order. Introduced in Generation V, Illusion is the signature ability of Zoroark and Zorua — only 2 Pokemon possess it, making it one of the rarest abilities in the game.