Generation Vii
Berserk
Boosts the Pokémon’s Sp. Atk stat when it takes a hit that causes its HP to become half or less.
Detailed Effect
Whenever this Pokémon takes damage from a move that causes its HP to drop below 50%, its Special Attack rises by one stage.
Pokémon With Berserk
3 canonical Pokémon forms can have this Ability.
Drampa 
Galarian Moltres 
Mega Drampa