Thirteen Knights of the Roman Catholic Church

According to Stiyl Magnus, the knights were created by the Roman Catholic Church after getting the idea from the Knights of England. Their primary purpose appears to be hunting down and eliminating threats and rogue magicians, like Aureolus Izzard

Deep Blood Arc
The knights were sent to take care of Izzard after he was discovered in Misawa Cram school. The initial strike did not go well, as at least one member was killed, as his corpse was found by Touma and Stiyl. The next night, the remaining members tried to destroy the building Izzard was in using a powerful spell. However, the spell was bounced back by Izzard using his Ars Magna.

Abilities
According to Stiyl, while the thirteen knights were based off of the Knights of England, compared to their English counterparts the Roman knights were second-rate amateurs. Even so, they were skilled enough to cast Grigorio's Chant, which normally would require 3300 monks to be used properly. They also appear to be quite good at stealth, as they were able to sneak into Academy City without the authorities knowing.

Trivia

 * It's unknown if the knights had the same duties as the Agnese Forces, though it seems likely.