Quivering palm

you gain the ability to set up lethal vibrations in someone’s body. When you hit a creature with an unarmed  strike, you can spend 3 ki points to start these imperceptible vibrations, which last for a number of days equal to your monk level. The vibrations are harmless unless you use your Action to end them. To do so, you and the target must be on the same plane of existence. When you use this Action, the creature must make a Con 15 saving throw. If it fails, it is reduced 2/3 Hit points. If it succeeds, it takes 3d10 necrotic damage.