The Solitaire’s Caress

Once per long Rest, when attacking a medium or smaller creature that is restrained by The Solitaire’s Embrace instead of striking with your dagger you can retract your blade and reach into your targets soul with your hand trying to sever it from its body.

If your target is below 100 HP it dies instantly. No saving throw, no death saves, no relentless endurance, no relentless rage, death. The pale lady awaits.
If your target is at 100 or more HP its soul cannot be severed fully, yet the procedure causes it immense pain and suffering. Roll psychic damage equal to your sneak attack bonus.