Shadow Tag
Prevents opponents from fleeing or switching out.
Full effect
While this Pokémon is in battle, opposing Pokémon cannot flee or switch out. Other Pokémon with this ability are unaffected. Pokémon with Run Away can still flee. Pokémon can still switch out with the use of a move or item.
In game description
The Pokémon steps on the opposing Pokémon's shadows to prevent them from fleeing or switching out.
Pokemon with Shadow Tag
Using Shadow Tag in Cobblemon
Abilities are passive and always active, so Shadow Tag 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.