I would argue that the "master" he refers to is Kelemvor, since Jergal has served several gods in recent history and Kelemvor is who he currently serves. His writing of those who pass on to the afterlife (both for mortals and gods, wherever they end up) might have been a task given to him by the gods he has served. This task might also be something just be a purpose that was given to him by another god or one his former-self gave to this lesser self after divulging his power.
