Spiritual Attachment

You’ve made a deal with a spirit, or had it latch on to you whether by choice or unexpectedly. Because of this, you are now cursed, haunted, if you will. With this haunting there may be power, but it will always come at a cost. It is even possible that the spirit takes over and gains control.

Whenever you reduce a creature to zero hit points, you may spend your reaction to gain temporary hit points equal to the damage dealt to reduce said creature to zero hit points. If you take these hit points, you must make a CON saving throw equal to 10 + (half the maximum of the temporary hit points). On a failed save, the spirit takes control.