The League of Elder: Sygillis of Metatron

Book 1 in the League of Elder Series.  When an enemy Black Hat is taken prisoner aboard Captain Davage's ship, the powerful Sisterhood of Light has every intention of executing her, to put her to a slow, painful death. That is what’s done to Black Hats, and no one dares challenge the Sisters’ authority.  They are astounded when the Captain refuses to allow it.  Though no one but a Sister would ever willingly face a Black Hat and hope to survive, he enters her cell, determined to discover the truth.