Master-Servant Vow (主従誓約 Shujū seiyaku) is what exists beyond the Master-Servant Contract.
The master servant vow is the deepest subordination one could give to one’s master. It’s the ultimate form of master-servant relationship, which could only be reached between a master who’s prepared for their subordinate to belong completely to them, and a subordinate who could vow their heart and body entirely to their master.
On the other side of the unbelievable power that came with it, is a forbidden relation that could not be erased. Among those whose souls are tied together by the master-servant magic, there aren’t many who could successfully reach the vow. It’s always been seen as nothing more than a legend or fairytale.
The vow ties master and servant together forever, and significantly boosts the power of a master as well as their servants. There are several conditions required for changing a Master-Servant Contract into a vow:
- The original contract has to reach its maximum possible state. Meaning the servant's loyalty has to be absolute.
- The master must request for the servants absolute and eternal loyalty. While the servant must willingly surrender body and soul to their master.
- Timing is the third condition. The servant’s loyalty has to be at its absolute limit, but it’s often difficult to assure that the servant’s purity would not have been taken before that limit is achieved. The exception to this is platonic master-servant relationships. Generally speaking, masters tend to take their servant’s purity before the time is right, during earlier stages of their master servant relationship. Even if the master and servants’ feelings are authentic and reciprocal, the servant wouldn’t have full recognition of their absolute loyalty to their master before the absolute limit was reached. This would create a scenario in which one side of the contract would be inadequate. Many master servant contracts are created through the master’s special abilities as such that their servants are usually cursed, and the forced nature of the contract prevents the servant from giving complete loyalty to their master. Abilities such as mind control or the hypnosis of the succubus and incubus make it impossible to reach the vow. They not only create distrust between master and servant, but the bond between master servant relationship is based upon a falsehood. Therefore in order to have a chance at successfully completing the vow, the bond between master and servant must be genuine. The master must also wait until after the servant has reached absolute loyalty, before taking the servant's purity.