Illusion will change the appearance of the Pokémon to that of the last Pokémon in the player's party. Illusion will not disguise the Pokémon if the last Pokémon in the player's party is fainted or is of the same species. Illusion will replicate the type of Poké Ball, nickname (or species name if none), Shininess (if any), gender, and form of the Pokémon it is masquerading as; however, it does not replicate the level of the masqueraded Pokémon. The effect is only aesthetic: effects like the move Attract still use the Pokémon's actual gender.
The illusion wears off when the Pokémon with this Ability is directly damaged by a damaging move or when Illusion is negated or replaced, causing the Pokémon with this Ability to appear as its true self. Indirect damage or loss of HP (such as through Substitute, Life Orb, weather, status conditions, entry hazards, recoil, and so on) will not break Illusion. If the Pokémon with Illusion is hit by a multi-strike move, the illusion will be broken after the first hit.
A Pokémon with Illusion can now disguise itself as another Pokémon with Illusion, provided that they are different species.
Role Play fails if the target has the Ability Illusion. Skill Swap fails if either Pokémon has Illusion. Illusion cannot be copied by Trace. If a Pokémon obtains this Ability with Imposter or Transform, it will not disguise itself.
Illusion can now disguise itself as another Pokémon with Illusion of the same species.
Illusion now disguises the Pokémon as the last conscious Pokémon in the party.
In Pokémon Mystery Dungeon: Gates to Infinity, the Ability functions similar to the main series games for foe Pokémon. A Pokémon with this Ability can transform into another Pokémon that can be found on the same floor of a Mystery Dungeon. For partner Pokémon, this Ability becomes nonexistent.
Zoroark's eyes glow light blue and its outlines start to glow pink. Then, a magenta wave of energy appears by its face and moves down to its feet. As the wave moves down, its body changes its shape into the opponent, or Zoroark's eyes glow red and it causes the opponent to see illusions of its choosing.
Zorua changes its appearance into something else, occasionally retaining its eyes, eyebrows, and eyelids when it does so. If Zorua takes damage, it takes a wisp-like appearance as it is forced out of its transformation.
In Generation V, if Entrainment is used on a Pokémon with Illusion while it is disguised as another Pokémon, when its Ability activates the banner will state that the Ability belongs to the Pokémon that was used as the disguise. If the Pokémon's Ability is changed again, these messages will correctly state the Pokémon who the Ability belongs to when it activates.