Ogerpon changes mask by holding a mask.
Ogerpon has three signature items, those being
- Hearthstone Mask
- Wellspring Mask
- Cornerstone Mask
and you need to make Ogerpon hold these masks to change it's form.
The standard Ogerpon is Grass type and you achieve it when it is
either holding no item or not holding a mask.
While wearing the Hearthflame Mask, Ogerpon turns into its Grass /
Fire form and switches abilities into Mold Breaker.
While holding the Wellspring Mask, Ogerpon turns into Grass / Water
and changes abilities into Water Absorb.
While holding the Cornerstone Mask, Ogerpon turns Grass / Rock and
switches abilities to Sturdy.
When Terastalizing while wearing the corresponding masks, they get a boost in stats and also can only Tera into their secondary type, and gains a new ability, Embody Aspect. If you're not using an item or not using a mask you will change to the Teal Mask form. (Hearthflame only gets Fire Tera, Cornerstone gets Rock Tera, Wellspring gets Water Tera)
Embody Aspect boosts a particular stat, depending on the form of Ogrepon:
- Teal Mask form: the Pokémon's Speed stat is boosted. (Grass Tera only)
- Heartflame Mask form: the Pokémon's Attack stat is boosted. (Fire Tera only)
- Wellspring Mask form: the Pokémon's Sp. Def stat is boosted. (Water Tera only)
- Cornerstone Mask form: the Pokémon's Defense stat is boosted. (Rock Tera only)
Source (Embody Aspect)
