A soul contract is a powerful binding that was created by the Sage Emperor to control members of the Sky Origin Divine Clan. In order to leave the Endless Wilderness, they must be bound to another with this contract. Using this, their owner can easily kill them if they want and they have absolutely no way of disobeying or escaping.[1]
Nie Li acquired many of these when he bought two hundred Sky Origin Divine Clan members from the Dragonbreath Chamber.[1]