The Pokémon changes form depending on how it battles.
Changes Aegislash to Blade Forme before using a damaging move, or Shield Forme before using King's Shield. Other-class moves other than King's Shield will not change this Pokémon's form, nor will any moves called by a move that calls other moves (e.g., Sleep Talk). The form change occurs even if the move causing the change would be totally prevented, e.g. even if Aegislash is asleep or hurts itself in confusion.
Aegislash reverts to Shield Forme outside of battle, including if switched out.
If another Pokémon somehow acquires Stance Change, it will not change form.
Pokémon with Stance Change as a standard ability (2)