Order of the Headless (Headless Monks)
The Headless Monks, or Order of the Headless, were a religious order who had themselves beheaded. They believed in listening to their hearts instead of their minds.
Biology
Since they were headless, they did not register as living beings. They couldn’t be fooled and were incapable of feeling fear. They could fire energy blasts from their hands as well as channel that energy into the swords they all carried.
Killing them was just as easy as killing a humanoid who had a head. They also were able to chat but it’s not known whether this was a form of vocal ability or psychic.
Culture
Once beheaded, they placed their heads inside a box-like device. These boxes were reserved for the very wealthy. The rest of the monks’ heads were left to rot on shelves. The heads and skulls were still “alive” in a sense.
If another group allied themselves with the monks, that group would have to provide at least one of their members to join the monks.
It was considered heresy to lower the hood of a Headless Monk. Since they followed an entity known as the Papal Mainframe, they were able to reveal their headless state to outsiders under the orders of the Mainframe.
Appearances
Prequel - A Good Man Goes to War (Webcast)
A Good Man Goes to War (TV)
The Wedding of River Song (TV)