Arma Haereticorum is a Mystic plan given to players who purchased the Diablo III: Reaper of Souls Digital Deluxe Edition or Collector's Edition. It provides three helmet transmogrification plans when used at the Mystic. They are:
This book of armor designs languished in Zoltun Kulle's archives for years. Following his resurrection and subsequent death, though, the Arma Haereticorum was retrieved from the desert sands and eventually found its way to you.