Lord Voldemort
Reasoning: This card stands for enslavement, ignorance, and giving into our darkest desires. Bleak and controlling, shallow and overindulgent. Voldemort symbolizes all that is evil in the Harry Potter world: his doctrine is one of fear and hatred towards Muggles and Muggle-borns, basing his entire regime on an ignorant and shallow perception of these groups. His Death Eaters are little more than fearful slaves, most of them are disposable to their master though they cling to his power and long for a bit of it for themselves. Death Eaters have given in to their hatred and lust for control, and in doing so lost power over their own lives by devoting themselves to Voldemort.
In a traditional tarot deck, this is the Devil.
Notes
Not much to say about this card. The original concept has remained unchanged through the numerous versions of this deck. Voldemort is fun to draw, especially his teeth for some weird reason... too bad they don't show as well as I would like in this picture. The Dark Mark was difficult to do by hand, but I think it turned out OK.