CURSED. the tiefling’s heart. its the actual thing believe it or not. After attunement, the withered heart acts on its on, it flys and slams itself into the chest of the person that attuned to it. Doing so it replaces the current heart. Changing the players race to Tiefling in an instant. All racial traits and racial proficiency bonus change to to that of the Tiefling. 

Notes: spend one long rest studying the heart