Special organs located inside the Magic Users chest in order to bond and become partners with one another, with the appearance of actual papers made of flesh.
They do not have any direct function in the body system aside of give extra censorial powers to know about the whereabouts and wellness of their partners, torn the contract will make the original owner to faint in pain.
They take a prominent role in the Magic User society during The Blue Night Festival, in which a Devil opens the chest of the sorcerers to exchange the organs with their respective partners once the sings are done.
Eventually the organ melts inside their new host and, if the contract was done by forcing a sorcerer to make the sign, it will bent their will and make it docile and obedient to his partner, this side effect makes the traffic of kidnapped sorcerers highly profitable and many people are forced to do this every four years.
The only way to escape from this effect is remove the contract by force from the body or if one of the involved parts die.