Listen up, true crime fans! Things are about to get dark on Oxygen as the network has announced its "deadliest event," 12 Dark Days of Serial Killers, airing April 9 through 20. Through two new specials, Catching a Serial Killer: Sam Little and Snapped: Notorious Hollywood Ripper, and the season premiere for Mark of a Killer and the season finale for The DNA of Murder With Paul Holes, Oxygen is producing the biggest event in true crime history.