A Promise Collar is a collar that, when entered into a contract, allows a master to be able to control the person wearing the collar. The collar itself has three commands: bind, sleep, and pain. 'Bind' temporarily disables the wearer so that they are unable to move for a certain amount of time, 'sleep' forces the wearer to sleep, and 'pain' causes the wearer to feel pain.
Overview
The promise collar when awake.
The promise collar is purple in colour with silver lining around it and a white band running through it. The collar's most distinctive feature is the yellow eye in the centre and the small mouth underneath it.
The promise collar enters into a contract by blood, when a person is bitten by the collar and the collar tastes the person's blood. Castor has said that only the Imperial army can remove the collars.
When Ayanami and Teito first fight in the Church, Mikage, possessed by Ayanami at the time, puts a promise collar on Teito. When the battle ends, Frau wants to help Teito to remove the collar, but he gets bitten by the collar, by which he becomes Teito's master accidentally.
In manga chapter 70, Ayanami captures Teito again and takes him back to the military. In chapter 71, Teito is seen without the promise collar.