Encroachment



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Pluses
The Encroachment is when a plus begins the process of transforming into a Hollow. Once the Chain of Fate is separated from a spirit, they can no longer return to their physical body. Death for the soul comes once the chain is completely eroded. One symptom right before the last erosion is an increase in appetite. The last erosion is much more shttp://bleachretribution.wikia.com/index.php?title=Encroachment&action=edit&useFormat=1ever than the others, as the final involved seven links of his chain, as opposed to the one link per erosion before it. They become Hollows afterwards, and have no chance of returning to their spirit form - the transformation is permanent, unless the soul is purified by a Shinigami's Zanpakutō so it can go on to the Soul Society. When a Plus descends into a Hollow, his/her spirit form shatters and reconstitutes into a different form. Then their Hollow Mask manifests.

Resistance
While the standard plus usually becomes a Hollow straight away, some are able to fight it if only long enough to defeat it. The transformation went completely out of sequence:Had the process continued it would have completely formed with no chance of returning to a plus. This ability to resist may or may not be implied for all souls.

People with Latent Shinigami Abilities/High Spiritual Power
The real test was not to escape the shaft, but for Ichigo to become a Shinigami while simultaneously preventing his/her own Hollowification - if a soul were to reach the encroachment and gain Shinigami powers in time, they can prevent their death.