See more: Chain
Chains are beings of the Abyss. At one point everyone of them was something with a living will but the power of the Abyss sinks into the being and they later become a Chain. If the being manages to stay alive, they evolve into a greater entity. The only way a Chain can escape from the Abyss is by contracting with a human, legally or illegally, and then escaping through the Way until they reach the outside world.