Aura Break
Makes Dark Aura and Fairy Aura weaken moves of their respective types.
Full effect
While this Pokémon is on the field, Dark Aura and Fairy Aura weaken moves of their respective types to 2/3 their power, rather than strengthening them.
In game description
The effects of “Aura” Abilities are reversed to lower the power of affected moves.
Pokemon with Aura Break
Using Aura Break in Cobblemon
Abilities are passive and always active, so Aura Break 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.