For Love Of Evil by Piers Anthony

A gifted but restless man is drawn into the office of Evil and must learn to fulfill the grim duties of tempting and corrupting souls while grappling with the moral ambiguities of his role; when unexpected love and his lingering human conscience complicate the tasks assigned by the cosmic bureaucracy, he confronts rivalries among the Incarnations, the nature of free will, and the costs of power, forcing choices that redefine both his identity and the balance between Heaven and Hell.

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