It is a mind controlling weapon and considered as "one of the great disasters" that were sealed away. It is a device that makes every subordinate act exactly as ordered.
It is currently housed at a shared control base by the British, German and French at the South Pole.
It is a radio device.
One Order was built by an ability technician, which was intended to be used by front line commanders to order their subordinates and execute the orders automatically. The concept of One Order was developed with the idea that armies wanted a "humane" tool to protect soldiers from war itself. Having some of the soldiers developed post-traumatic stress disorder and suicide became frequent, as a result of the army training to "shoot by reflex, without thinking". 
When a subordinate is commanded by One Order, their body moves like an extension of their superior's, and executes the orders automatically.