Master-Servant Vow (主従誓約 Shujū seiyaku) is what exists beyond the Master-Servant Contract.
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.
- Servants have to be willing to devote all of their heart as well as their body eternally to their master