Stance Change
Changes Aegislash to Blade Forme before using a damaging move, or Shield Forme before using King’s Shield.
In game description
The Pokémon changes its form to Blade Forme when it uses an attack move and changes to Shield Forme when it uses King’s Shield.
Pokemon with Stance Change
Using Stance Change in Cobblemon
Abilities are passive and always active, so Stance Change costs you nothing to run. If it is marked hidden on a species above, expect it to be harder to obtain: servers differ in whether hidden abilities come from breeding, an Ability Patch, or events.
See the Pokedex for which abilities each species can have, and IVs, EVs and breeding for how to actually pass one down.