Mark Rein-Hagen is a role-playing, card, video and board game designer best known as the creator of and its associated World of Darkness games. Along with Jonathan Tweet, he is also one of the original two designers of Ars Magica. His work on World of Darkness has influenced the movie series Underworld, True Blood, and otherkin sub-cultures such as Real Life Vampires and Real Life Werewolves.