Aftermath is an ability that only is on the Voltorb and Electrode line in Polished Crystal. When a Pokémon with Aftermath is knocked out by a move that makes contact, the attacker loses 1/4 of its maximum HP.
This ability can be useful for punishing physical attackers who make contact, especially if they are already low on health.
This ability is only found as a Hidden Ability, so finding a Voltorb or Electrode with Aftermath may require some effort. Try using Hidden Grottoes or Ability Capsules to obtain it!