Order of the Headless

The Headless Monks were a religious order who believed in listening to their hearts rather than their minds, to the point of having themselves beheaded.

Biology
As they were headless, the Headless Monks did not register as living beings. They were also supposedly incapable of being fooled or feeling fear. Despite their lack of heads, the Monks were able to detect where other beings were. They could also fire energy blasts from their hands, similar to the energy of the swords they carried.

They could be killed just as easily as most other humanoids, with energy blasts, guns, or sword wounds. They also had some form of vocal ability, as they could chant. (DW: A Good Man Goes to War)

Culture
The Headless Monks were very religious. They believed in listening to their hearts rather than their minds, so they beheaded themselves, and placed their heads inside an unnamed, box-like device. If another group allied with the monks, they were expected to provide at least one of their members to join the monks.

It was a Level One Heresy, punishable by death, to lower the hood of a Headless Monk. The group followed an entity known as the Papal Mainframe. Although the monks considered it a heresy to reveal their headless state to outsiders, they were apparently able to do so under the orders of the Mainframe.

The Monks would perform an attack chant before certain fights. (DW: A Good Man Goes to War)

History
The Headless Monks were old customers of Dorium Maldovar. They paid him for delivering them the brain of a Judoon. (DW: Prequel (A Good Man Goes to War)) They were also present at Demon's Run when they fought the Eleventh Doctor and his allies. During this time they killed several people, including Maldovar, but eventually all present were killed. (DW: A Good Man Goes to War) The Delirium Archive was said to be their final resting place. (DW: The Time of Angels)