Stance Change is Pokémon ability exclusive to Aegislash that allows it to switch between its formes.
The ability allows Aegislash to switch between its Shield Forme, which has high defensive stats but low offensive stats, and its Blade Forme, which has high offensive stats but low defensive stats. Aegislash will turn into its Blade Forme when it uses a damaging attack, but can only turn back to its Shield Forme if it uses King's Shield. Other status moves will keep the current Forme active. Even moves like Sleep Talk activating an attack move or King's Shield will not trigger a Forme change.
The Ability activates prior to and independently of any checks for status conditions that may stop the user's attack, such as sleep or confusion.
Stance Change cannot be Traced, copied with Role Play, suppressed with Gastro Acid, or replaced with Simple Beam, Worry Seed, Entrainment, Skill Swap, or Mummy. If a Pokémon other than Aegislash obtains Stance Change with Imposter or Transform, the Pokémon will not change Formes.