Mummy

Mummy is a contagious ability introduced in Generation V that overwrites the attacker's ability whenever they make contact with the Mummy user. The replaced ability is permanently lost for the duration of the battle (unless the affected Pokemon switches out and has its original ability restored upon re-entry, depending on the ability).

The contagion effect chains -- once an attacker gains Mummy, any Pokemon that makes contact with them also receives Mummy, potentially spreading across an entire team. This makes physical attackers hesitant to touch Mummy users, as losing their own ability (such as Huge Power, Adaptability, or Intimidate) can be devastating. Certain abilities are immune to replacement: Multitype, Stance Change, and other unreplaceable abilities cannot be overwritten by Mummy.

Mummy is found on only 2 Pokemon: Cofagrigus and Yamask. An important edge case involves the interaction with Moxie -- if a Moxie user KOs a Mummy Pokemon through a contact move, Mummy replaces Moxie before the KO boost would activate, meaning the attacker receives neither the Moxie Attack boost nor retains their original ability. In competitive play, Mummy acts as a deterrent against physical contact-heavy teams, punishing switch-ins and pivots that rely on their abilities for power or survivability.