Tera-Shell is an ability introduced in Pokémon Scarlet and Violet, exclusive to Tera Pokémon, a unique subset introduced with the new Terastal phenomenon in the Paldea region.
Tera-Shell grants the Pokémon immunity to all damage from its primary type while it is in its Terastal form. For instance, if a Water-type Pokémon with Tera-Shell terastalizes into a Fire-type, it becomes immune to Fire-type moves for the duration of the Terastal transformation.
Increased Damage:
Pokémon with this ability also gain a boost to the damage of moves that match their Terastal type. This boost is usually equal to the same type attack bonus (STAB) but applies additional multipliers due to the Tera-Shell's unique properties.
Stat Boosts: