Greetings, aspiring demon lords! Welcome to How to Cuck Humans into Servitude 101, also known as Demonic Contracts!
First, demonic contracts are only available to pureblooded demons. It's a race privilege, not a strength privilege. So, half-demons, fiends, and demonic beasts cannot use them, regardless of cultivation.
(Nephilims are the exception)
Also, a contract is always a willing agreement between parties. Regardless of the method used (blackmail, coercion, threatening, etc…), the target still needs to agree to the terms (to some degree) before the contract can pass.
For that reason, Phantasm Orchids are not contracts. They are mind-control tools. Konrad can use them to form contracts by proxy if the two parties agree (like with Margo, for example) but just swallowing it isn't an automatic contract.
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Now, let's see the actual contracts, from highest to lowest:
A) Partnership or Equals (Also known as the invitation into the household): This is the best contract a demon can offer, and also the most dangerous one because it can only be broken when one party infringes on the terms.
Through this, the demon and the target exchange boons such as talent, abilities, and physiques. The contractor then injects his blood into the target to cement the deal. In that instant, he can choose to grant them a portion of his bloodline (at best, one level below).
Thus, although this contract provides mighty boons, it makes it very difficult to control the contracted. It relies entirely on trust, and there are quotas depending on bloodline level.
Note: Throughout the three realms, humans are the only race capable of turning into anything. Konrad can use his blood to purify the lineage of demon descendants and help them complete an Atavism. But he cannot turn a winged serpent into a succubus, for example. Only the system can.
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B) Master-Servant Contract (Or Household Slave Picking):
This can be the best or the worse contract. Once signed, the contracted is at the absolute mercy of the contractor. Their memories, talent, soul, blood, lives, everything they are and have, belongs to the contractor. He can do with them as he sees fit.
For that reason, no matter where they stand in the multiverse, the contractor can summon them by his side, and they cannot resist the call.
At the same time, there is no limit to the boons they can receive. The contractor can keep giving them additional rewards without any constraints whatsoever. Therefore, their position in the household often depends on how much the demonic master dotes on them. They also possess their master's house's Mark of Servitude, which in itself is a proof of identification.
Not many dare disrespect the servants of mighty Demon Gods such as the Four Kings and other Primogens.
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C) Mutual-Obligation Contract (Or Indentured Servitude): Often used by Demon Gods to test potential servants.
This contract is an exchange of services. The contractor offers to grant the contracted's wishes on the condition that they fulfill his demands (For example, Talroth and Gulistan, Konrad and Astarte). It can also be a payment for rendered services.
Once the obligations are fulfilled, the contract ends. But there are often terrible consequences for the contracted's failure.
Note: The Mutual-Obligation Contract is in demonic language. Therefore, it's very simple for the contractor to add additional closes and screw over the contracted (if they don't know the demonic language).