Shadow Shield
When this Pokémon has full HP, regular damage from moves is halved.
Full effect
When this Pokémon has full HP, regular damage (not fixed damage!) from moves is halved. This ability cannot be nullified.
In game description
Reduces the amount of damage the Pokémon takes while its HP is full.
Pokemon with Shadow Shield
Using Shadow Shield in Cobblemon
Abilities are passive and always active, so Shadow Shield 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.