Any damage that Strahd takes is transferred to the Heart of Sorrow. If the heart absorbs damage that reduces it to O hit points, it is destroyed, and Strahd takes any leftover damage. The Heart of Sorrow has 100 hit points and is restored to that number of hit points each dawn, provided it has at least 1 hit point remaining. Strahd can, as a bonus action on his turn, break his link to the Heart of Sorrow so that it no longer absorbs damage dealt to him. Strahd can reestablish his link to the Heart of Sorrow as a bonus action on his turn, but only while in Castle Ravenloft.