Berserk
Raises this Pokémon's Special Attack by one stage every time its HP drops below half.
Full effect
Whenever this Pokémon takes damage from a move that causes its HP to drop below 50%, its Special Attack rises by one stage.
In game description
Boosts the Pokémon's Sp. Atk stat when it takes a hit that causes its HP to drop to half or less.
Pokemon with Berserk
Using Berserk in Cobblemon
Abilities are passive and always active, so Berserk 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.