When a character has something physically happen to their heart (often in a way that generally would be impossible without magic being involved), it often affects their personality in drastic ways. Either they lose all of their emotions, or their personality changes--for the worse. Seen most often in fantasy works, particularly those with a symbolic or Fairy Tale theme. This trope differs from The Heartless in that while those characters are simply composed of negative emotions (which might not actually be related to their hearts), these characters sometimes literally lose their hearts.