While slaying undead in the Scarred Mire one of them will drop a Muddy Journal. The first step of this quest will be to slay Abelus Crane in the Scarred Mire. You can find him due west of the map marked for the Scarred Mire on your region map. After laying Abelus to rest, again, head to Meridian in search of Sara Martins.
Sara Martins can be found in the north section of Meridian, near the Porticulum.