Master-Servant Vow

The Master-Servant Vow (主従誓約, Shujū Seiyaku) is what exists beyond that of even the Master-Servant Contract.

Description
"When it then raises to the highest level―maximum loyalty, the mark disappears and the curse won't activate anymore. After all, the servant won't ever betray the master at this point. That's the phenomena where the 'contract' sublimates into a 'vow'. Though there are not many cases of reaching that state, even from the past. But it's said that no one was able to win against a master and his servant in that state."

- Nico Rasalgethi explaining the Master-Servant Vow The Master-Servant Vow is the deepest subordination that a Servant is able to give to their Master. Its the ultimate form of Master-Servant relationship, exclusive to both the basic contract and Evils Pieces, which exists between both a Master whose prepared for their subordinate to belong to them fully, and a Servant whose able to vow their heart and body entirely to their master. Nico and Lars has stated that there haven't been many high or ultimate-ranked devils able to reach this level of the Master-Servant Contract even among the ones who created it in the past, possibly due to the trust that is required from both parties involved in the process, but it was said that no one will be able to win against a Master and Servant who achieved this state.

On the other side of the unbelievable power that comes with achieving this, is a forbidden relationship that can't be erased. This also causes the collar, which represents the Master-Servant Contract to vanish since there isn't a need for their servant to be cursed anymore.

Purpose
The Master-Servant Vow ties master and servant together forever and significantly boosts the power of the master as well as their servants involved. Additionally, each additional contract that reaches the vow state also further boosts the power of both master and servant.

There difficult conditions required to change a Contract to a Vow:


 * 1) The original Master-Servant Contract has to reach its maximum possible state, meaning that the Servant's loyalty has to be absolute as signified by the crimson red collar.
 * 2) The Master must request for the Servants' absolute and eternal loyalty, while the Servant must surrender their body and soul to their Master.
 * 3) Timing is the third and final condition. Since the Servant's loyalty has to be at its absolute limit, it is at times often difficult to assure that their purity wouldn't be taken before that limit is achieved. The exception is that their relationship must be platonic and the master must take their servant's purity before the time is right in the middle of the earlier stages of their 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, which would cause one side of the contract to be inadequate. Due to the initial contract being formed using the master's or someone else's unique nature their servants' are cursed, and the forced nature of the contract prevents the servant from giving complete loyalty to their master.
 * 4) *As such, those who formed the contact using succubus' or incubus' natures, which involves both mind control and hypnosis makes it near impossible to reach their vow. It creates distrust between Master and Servant, due to the relationship being based on falsehood. So in order to succeed in completing the vow, their bond must be genuine. The master must also wait until after the servant has reached absolute loyalty before they take the servant's purity.

Trivia

 * The Master-Servant Vow could be the end result of what the Ancient Devils meant to reach, but ironically couldn't as it required trust and loyalty on both sides.
 * Whenever Arthur or Lancelot gets turned on and begins having sex with the girls in their respective harems, despite the collar being removed, the collars and succubus curse activates.