The Pardoner's Oath is a vow made by Apprentice Pardoners before full initiation into the guild. Given that Pardoners actively engage Shadows, they have almost unlimited potential for blackmailing Wraiths. Historically, Pardoners maintained a partnership with the Mnemoi in order to erase memories that they did not wish for their own Shadows to have access to.
The Pardoners maintain their integrity through the maintenance and strict enforcement of a specific Oath which resembles a hybrid of the Hippocratic Oath and the seal of the confessional. Pardoners keep everything they are told when engaging in Castigate under strict confidence, and even use special Castigation arts to shield that knowledge from their Shadows.
Pardoners take this Oath very seriously, as does the Hierarchy. Internally, the Guild refuses to teach anyone who refuses to take the oath, and violating the oath leads to censure and ostracism.