[虚淵 玄] (うろぶち げん)
Japanese writer and scriptwriter. Also known as "Urobutcher" due to his many of his stories having dark and nihilistic themes and/or tragic plot twists liberally sprinkled with gory elements, such Fate/Zero, Phantom of Inferno, and Mahou Shoujo Madoka Magica. See post #1424416 for some examples of stories he's left his signature touch on.
Wikipedia article: http://en.wikipedia.org/wiki/Gen_Urobuchi