Contract

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Contract is a binding verbal agreement between a Devil and a Human.

Overview
Contracts can only be formed between devils and humans. Thus, fiends are incapable of performing contracts. Contracts are binding in nature and cannot be broken on pain of death. When a devil offers a contract to a human and the human accepts, neither can go back on their word. Despite this it is implied that an "exit clause" of sorts can be added for peaceful termination of the contract, seen when the Fox Devil terminated his contract with Aki yet neither died.

By sacrificing something the Devil finds valuable and will cause suffering, humans are able to use the supernatural abilities of contracted devils. Humans can sacrifice their body parts, senses, lifespans, legal children or their legally responsible individuals (for governors etc.) by verbally accepting or offering the terms. Although they cannot sacrifice someone unrelated to them, they can incapacitate someone else to offer them to a Devil, as seen when Santa Claus is able to offer an incapacitated Denji by bringing him to Hell with one of her perfect dolls which also indicates that a proxy is adequate to perform legitimate contract.

Devils are able to take their sacrifices by seemingly making them vanish instantly, as seen when the Ghost Devil takes all of Himeno. Sacrifices can also be taken regardless of distance, as seen when the Hell Devil took Santa Claus' children while they were in Germany. It is unknown what happens to a sacrifice after being taken; though Himeno's "easy revenge" cigarette implies they aren't simply destroyed.

Humans and their contracted devils can communicate with each other by being summoned as seen with Aki Hayakawa and his contracted devils, the Curse Devil and Future Devil, and with Himeno when she talked to the Ghost Devil's hand. They can also learn how much of a sacrificed resource they have left, such as lifespan, it if thats the cost of the contract.

Devil Hunters tends to hide the details of their contracts and the abilities of their contracted devils to prevent their enemies from learning their weaknesses and strengths, as shown by Kobeni Higashiyama. Pingtsi is able to see what Devil contracts a human has by transforming her futakuchi-onna-like hair ponytail of hers into a monocle/magnifying glass and looking through it.

It is shown that in Japan only Devil Hunters, either private or from Public Safety, can legally make contracts; presumably, similar policies are in place around the world.

Devil Transformation
A type of contract where a human can gain a monstrous or very inhuman form is called a "devil transformation". This appears to be different from the contract with hybrids and the forms that have been seen appear even more inhuman than them. As devils fused into hybrids will gain a mostly permanent fusion with the human who became one, devil transformations seem to be a human's own transformation derived from the devil they contracted with but without the same exact function or triggers shown like a hybrid. A person's own individual devil transformation can look unique even if those individuals contracted with the same exact devil. The people who have devil transformations that we've seen so far are: Santa Claus' contract with the Doll Devil and later the Darkness Devil, Class President and finally Yuko's transformations from the Justice Devil. It is unknown how many people with a devil transformation can easily turn back into their human form as Yuko said that she became a "devil" and was trying to find her devil hunter relative who she said can turn her into a human again.