Multiattack. The mummy makes two Rotting Fist attacks and uses Dreadful Glare.

Rotting Fist. Melee Attack Roll: +5, reach 5 ft. Hit: 8 (1d10 + 3) Bludgeoning damage plus 10 (3d6) Necrotic damage. If the target is a creature, it is cursed. While cursed, the target can't regain Hit Points, its Hit Point maximum doesn't return to normal when finishing a Long Rest, and its Hit Point maximum decreases by 10 (3d6) every 24 hours that elapse. A creature dies and turns to dust if reduced to 0 Hit Points by this attack.

Dreadful Glare. Wisdom Saving Throw: DC 11, one creature the mummy can see within 60 feet. Failure: The target has the Frightened condition until the end of the mummy's next turn. Success: The target is immune to this mummy's Dreadful Glare for 24 hours.