The Prefecture Magisterium (also known as the Malagra) is a division of the Adeptus Mechanicus, acting as its de facto secret police. It is charged with preserving the dogma of the Cult Mechanicus and the persecution of Hereteks. It operates military Cohorts and hunter-killer clades which are maintained on Forge Worlds.
Agents of the Prefecture Magisterium are known as Arcuitors and are usually Magi themselves. Known for their devoutness to the Cult Mechanicus, these sinister figures are also dubbed the Paternis Malagra or Fathers of the Malagra, for they are often ancient beyond belief and heavily augmented. The Arcuitors do not officially lead the Prefecture Magisterium, but through their actions are the agendas of the organization shaped. A senior Arcuitor can call upon great resources against those they deem worthy of persecution such as the Ossifram Majority or the Dark Mechanicum.