Encroachment

The Encroachment is the process of a Plus losing the remnants of his Chain of Fate. When the final links in the chain are eaten away, the Plus essentially loses its heart, a hole is formed in its chest and it is transformed into a Hollow.

Overview
The process phisycally begins the moment the body dies and the Chain of Fate is broken. Humans Souls who have been removed from their bodies without the chain being broken can normally be returned to their bodies. The Encroachment usually lasts months or even years, although a special gas can speed up the process to a mere 72 hours. With every cycle of the Encroachment the furthest link in the chain begins to gnash and chew into itself and the links attached to it. The process cannot be halted by any known means - any attempt to do so would end in a nasty bite. During each cycle the Plus experiences excrutiating pains, the worst of whice come with the final cycle which is much more powerful than those before, wherein all the remaining links undergo the Encroachment at once.

Shattered Shaft Lesson
During the Trials of the Shattered Shaft, Ichigo Kurosaki was able to overcome the process of Hollowfication with the aid of Zangetsu, who pulled him into his internal world and taught him how to tap in to his latent Shinigami powers. However, since Zangetsu's intervention came after the Encroachment was physically through, an inner Hollow, similar to those of the Visored, was formed in Ichigo's internal world, manifested by a Hollow mask created over Ichigo's face.