The Sin

The Sins serve and protect the crown. That is their only purpose and they are the last line of defense. They were formed early on during the first centuries of the kingdom's formation, but their date and reason of origin are unknown even to most of the Sin.
The people of this group are the marks of a Sin. They are the crown's personal assassins. They sin so the crown can reign with a pure conscience. They are taken at an early age from families with too many children. Only the youngest child is taken and they ensure that the family's linage will continue by refusing to take the oldest children. A family can only offer one child to the Sin and the offer must be done before the child turns 10.
At ten, children are recruited to a guild nearest to their home. There, they are tested and trained for two years until they are tested to see if they can become a Sin. If the child succeeds, they are given a dot just below the eye. The Sin's earn dots based on their successes. The first is earned when you become a sin. Sin Brothers earn black dots on their right eye and Sin Sisters earn red dots under their left eye. (Gotta find a way to fuck around with the binary)
The first is given to you after your training is complete and you are officially recognized as a Sin. The second is earned when the Sin completes their first assassination. With the second dot, they become a Sin Sister or Brother. The last is hard to earn. It’s when a Sin is given a direct assignment from the crown.
The hardest to earn are the dots of the opposite cheek. Traditionally, a Sin can only gain three dots in a lifetime, but on occasion, Sins are given more dots to represent outstanding loyalty and bravery in the Sin Guild. The Sin Masters, the leaders of the guild, have to gain four dots before they are eligible to become a Master. Four is uncommon, five is rare, and six dots is unheard of in the guild. While a few will gain four during one king's reign, maybe one will gain five during that time. Six hasn't been recorded in centuries but it is something all Sins strive to gain.
The Sins are the crown's shadow and do all that the people should not know of to ensure the crown can rule without worry. Over the years, while still powerful, they have become leashed by politics and have lost most of their control. They still carry out their basics functions, but they have to complete with nobles more to be able to complete their tasks.