From Lexicanum

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Penitents (also known as bound psykers, penitent witches, and pariahs) are rogue psykers that have been captured by an Inquisitor and subsequently confessed to his or her crimes. Doing so allows the psyker to escape execution, and obligates him to a lifetime of atonement. The psyker is then bound with wards and anointed with sacred oils, to ensure that it does not regress to its previous heretical ways.

Unfortunately, it is rare to find a rogue psyker truly repentant for his crimes; but to do so is a high aspiration and goal for many Inquisitors of the Ordo Hereticus.

Uses

Penitents act as "psychic lightning rods", attracting and then dissipating psychic attacks. Because of this protection, they are often used by Inquisitors in the field as part of their retinue.¹

Sources

• ¹Codex: Witch Hunters (3rd Edition) pg. 15