Edward Mordrake was a 19th-century English man who (according to an urban legend) had an evil little face on the back of his head.
Although the face could not see, eat, or talk, it had an agenda of its own and could whisper (only to Mordrake), laugh, and cry and drool.
Mordrake claimed that the face whispered bad things to him at night. He also complained that the face would sneer when he was happy, and smile when he was crying.
Mordrake pleaded with doctors to remove the "demon face," but they refused to attempt the operation. Mordrake was so tortured by the face on the back of his head, that he killed himself at age 23.
Although Mordrake's story is likely just a creepy myth his condition was described in Anomalies and Curiosities of Medicine:
One of the weirdest as well as the most melancholy stories of human deformity is that of Edward Mordake, said to have been heir to one of the noblest peerages in England. He never claimed the title, however, and committed suicide in his twenty-third year. He lived in complete seclusion, refusing the visits even of the members of his own family. He was a young man of fine attainments, a profound scholar, and a musician of rare ability. His figure was remarkable for its grace, and his face – that is to say, his natural face – was that of an Antinous. But upon the back of his head was another face, that of a beautiful girl, "lovely as a dream, hideous as a devil." The female face was a mere mask, "occupying only a small portion of the posterior part of the skull, yet exhibiting every sign of intelligence, of a malignant sort, however." It would be seen to smile and sneer while Mordake was weeping. The eyes would follow the movements of the spectator, and the lips "would gibber without ceasing." No voice was audible, but Mordake avers that he was kept from his rest at night by the hateful whispers of his "devil twin", as he called it, "which never sleeps, but talks to me forever of such things as they only speak of in Hell. No imagination can conceive the dreadful temptations it sets before me. For some unforgiven wickedness of my forefathers I am knit to this fiend – for a fiend it surely is. I beg and beseech you to crush it out of human semblance, even if I die for it." Such were the words of the hapless Mordake to Manvers and Treadwell, his physicians. In spite of careful watching, he managed to procure poison, whereof he died, leaving a letter requesting that the "demon face" might be destroyed before his burial, "lest it continues its dreadful whisperings in my grave." At his own request, he was interred in a waste place, without stone or legend to mark his grave.
It's unfortunate that we don't get to find out what Mordrake's life would have been like past age 23, but If I had an evil face on the back of my head that smiled when I wept, I would gladly self-destruct.