Ability₁₀₇-Illusion

⚠️When a Trainer's Pokémon with Illusion enters battle, Illusion changes the appearance of the Pokémon to that of the last conscious, non-Egg Pokémon in its Trainer's party.
⚠️If the Pokémon with Illusion was the last conscious non-Egg Pokémon in the party before being switched in, Illusion does not change its appearance.
⚠️Illusion replicates the appearance (including species, form, and Shininess), nickname (or species name if none), Poké Ball, and gender of the Pokémon it is masquerading as; however, it does not replicate the level of the masqueraded Pokémon.
⚠️The effect is only aesthetic-effects such as Attract still use the Pokémon's actual gender.
⚠️On the user interface, an opponent's Pokémon disguised by Illusion will appear to be the type of the Pokémon it is disguised as (even if it is affected by a move such as Soak).
⚠️The illusion disappears if the Pokémon has its Ability changed or suppressed or if it takes damage from a damaging move.
⚠️Indirect damage (such as from a Life Orb, weather, status conditions, recoil, etc.) will not break the illusion. If the Pokémon with Illusion is hit by a multi-strike move, the illusion will be broken after the first hit.
⚠️If a Pokémon with Illusion uses a Z-Move, the illusion wears off immediately before using the Z-Move.
⚠️Role Play fails if the target has the Ability Illusion.
⚠️Skill Swap fails if either Pokémon has Illusion.
⚠️Illusion cannot be copied by Trace, Power of Alchemy, or Receiver.
⚠️If a Pokémon obtains this Ability with Imposter or Transform, it will not disguise itself.
⚠️Pokémon cannot transform if disguised by Illusion, nor can it transform into a Pokémon that is disguised by Illusion.
⚠️Additionally, wild Pokémon can disguise themselves with Illusion under certain circumstances:
☯️In Pokémon Black and White, in the special event encounter with a wild Zoroark in Lostlorn Forest, the wild Zoroark will be disguised as Entei, Raikou, or Suicune (depending on the Pokémon that triggered the event).
☯️If a wild Pokémon with Illusion successfully calls an ally for help in an SOS Battle, it will disguise itself as the summoned ally.
🔵Gen⁵
If Illusion would disguise the Pokémon as another Pokémon of the same species, it fails to disguise the Pokémon at all.
🟣Gen⁶
Illusion can now disguise itself as another Pokémon with Illusion of the same species.
🟢Gen⁸
If the Pokémon Dynamaxes while disguised as a Pokémon with the Gigantamax Factor, it will Dynamax into the standard form of the Pokémon, not the Gigantamax form.
🚫If disguised as Zacian, Zamazenta, or Eternatus, the Pokémon will not be able to Dynamax.
🔴Gen⁹
In most circumstances, if the Pokémon Terastallizes while disguised, it will not lose its illusion. However, it will Terastallize into its Tera Type rather than the disguised Pokémon's Tera Type.
🚫If disguised as Ogerpon or Terapagos, the Pokémon's illusion will fade before it Terastallizes.
⚠️If the last conscious non-Egg Pokémon in the party is a Terastallized Ogerpon or Terapagos, Illusion will not activate when the Pokémon is switched in.
⚠️Illusion will also not activate if the Pokémon with Illusion is Terastallized and the last Pokémon in the party is Ogerpon or Terapagos.
⚠️If the Pokémon is out of its Pokeball and the player starts a battle with a wild Pokémon, Illusion will not activate at the beginning of the match.
⚠️Illusion will now always be used by Wild Zorua and Zoroark to disguise themselves as another Wild Pokémon in the area, unless the game is reloaded when one has already been spawned. However, it wears off as soon as the battle starts, with the standard Ability activation message popping up.
~Bulbapedia
-------------------------------------------------------------------------------------------------------------
Worry Seed works on Illusion turning it into Insomnia
(regardless of whether Zoroark is disguised or not).