Ok, the background looks ok and interesting idea that reason hate the undead is sake of cheating of fairness of death. Anyways even if Ian has zero religion lore (which he should not have as it's useful skill to know undead abilities), he should be aware of Kelemvore (https://forgottenrealms.fandom.com/wiki/Kelemvor), god of death. The ultimate judge who decides where souls go (based on piety shown but also personal deeds and beliefs). Those without faith go to wall of faithless, which is<em>worst</em>possible outcome to any mortal being. As you are literally transformed into a brick made from soul and form into one piece of wall around gloomy city of the dead (and keep souls and dead where they belong).<br/><br/>Also your story can work anywhere in Faerun so location is not important as anywhere there are nobles who abuse their power one or another. However Tethyr is land which fairly recently made massive civil war where nobles were ousted/killed on sight and wiped out many noble bloodlines.<br/><br/>Another part vampires in general are hardcore undead so I am fine if you have killed vampire spawns or recent vampires (so nothing famous ones). But 13 is quite a lot for 5th level (assuming he did killing alone or part of small parties in the past).