Cancellarius

A Cancellarius (隠秘記録官(カンセラリウス)) is a term used to refer to a position in the Roman Catholic Church whose primary task is to decode modern witchcraft. Aureolus Izzard is currently the only character known to have ever been in the position.

Etymology
The name is derived from the Cancellarii, a lattice-work placed before a window, a door-way, the tribunal of a judge, or any other place. Hence the name for officials that stood at the latticed or grated door of the emperor's palace.

Moreover, the chief scribe or secretary was called Cancellarius.

Duties
The duties of the Cancellarius is to decode modern witchcraft, find ways to counter them and record them down in books. Specifically, make grimoires to counter magic that the Church doesn't allow. However, even though it's a grimoire, the purpose is of the complete opposite. It's like some teaching guide for "which line in the Bible can be used to handle the spells those witches use," according to Stiyl Magnus.