Golbat

Until the end of your next turn, if an attack damage the Defending Pokémon (after applying Weakness and Resistance), that attack does 20 more damage to the Defending Pokémon

If this attack damage the Defending Pokémon, the Defending Pokémon is now Poisoned and you remove a number of damage counters from Golbat equal to half that damage (rounded up to the nearest 10). If Golbat has fewer damage counters than that, remove all of them. Either way, the Defending Pokémon is now Poisoned.