Chain of Fate

Every departed spirit carries a Chain of Fate, the cord that once joined it to a beating body. Where the soul's lingering attachments point decides its fate: a spirit clinging to a particular person has its chain fixed to that individual and lingers as an obsessed spirit, or Tsukirei, while one whose death or grief is tied to a place becomes an earthbound spirit, or Jibakurei, its chain rooted to that spot.

Once a person's spirit is torn from its body, the exposed chain radiates intense agony, often tempting its owner to snap it. Doing so is catastrophic, since a severed link bars the soul from ever slipping back into its flesh. A Shinigami can still reunite spirit and body through healing Kidō, but only while the chain stays whole and the two have not been parted for long.

The erosion itself is called Shinshoku, or Encroachment, and it begins the instant the body dies and the chain breaks. Left alone it can grind on for months or years, though a specialized gas compresses the whole ordeal into roughly three days. In each round of decay the chain's outermost link bites and devours itself along with those beside it, a process no known method can halt, since interfering only earns a vicious bite. Every cycle brings searing pain, and the last is the cruelest, striking all the surviving links at once and completing the change into a Hollow.

Every Plus in the World of the Living carries a Chain of Fate, and Ichigo Kurosaki nearly lost himself to a Hollow when his own chain underwent forced Encroachment. A Shinigami possesses a counterpart to it: though no visible cord remains, the spot where the chain was anchored survives as the Saketsu, or Binding Chain, which works together with the Hakusui, the Soul Sleep that generates spiritual power. Both act as pressure points, and piercing them seals a Shinigami's strength away permanently, as Byakuya Kuchiki demonstrated by using Senka to strike Ichigo's Saketsu and Hakusui.