Craniopagus Parasiticus – it sounds like a Harry Potter spell and if it were, it would be one of the more insidious. Those afflicted with it are born with a parasitic twin head. I’ll repeat that in case you have one of your own. Parasitic. Twin. Head. And not in the fashionable mode of Zaphod Beeblebrox. Rather, imagine a duplicate head growing from the crown of yours but (usually) upside down as if you drew a face card from the deck and creased such that its top and bottom met.
Craniopagus Parasiticus is the result of a conjoined twin, in utero, failing to develop from the chin down. Truly the stuff of nightmares – especially if you had the bad luck to be 19th century Englishman Edward Mordake who is purported to have had a second face on the back of his head. Apocryphally, the face was was that of a woman, which would seem impossible since conjoined twins are necessarily identical.

The face neither ate nor spoke but would laugh and cry. Mordake also claimed it whispered Satanic incantations to him at night. Stories of Mordake’s case are difficult to verify though gallant attempts have been made in the blogosphere, most cogently by Curious Tendency and by Verum Fabula.

Why is this so fascinating? Besides the arresting eye-grab of the images themselves, two-headed or two-facedness provides a convenient metaphor for dualities in our nature. Add to the fact that one head is described as “parasitic” (second heads draw more blood from the circulatory system than their owners’ hearts can often tolerate), all but demands us to perceive the second head as evil. Naturally, this makes for delicious narrative fodder. At least Tom Waits thought so when he penned a ballad about Mordake’s fate entitled “Poor Edward.” It it, Waits’ narrator conjectures that Edward’s second face was indeed that of a woman who led Edward to take his (and her) life:

Some still believe he was freed from her
But I knew her too well
I say she drove him to suicide
And took poor Edward to hell

Here’s an interesting claymation fan video by Kanal von ElKulte:

Poet Shel Silverstein explored a similar situation in his poem Us in which one of two-faces affixed to a floating head bemoans the contrary nature of his twin. Unlike Poor Edward, Silverstein’s poem ultimately concludes philosophically:

But I guess there’s worse things
We could be –
Instead of two we could be three,
Me and him
Him and me.

Two heads are not better than one but they’re apparently better than three.