The Dead wars started with a plague like the black plague. In addition to causing nasty illness, it also diminished the victim’s intelligence. So many people died from the illness and then died from stupidity. Once they died from stupidity, they turned into mindless or mind-controlled zombies. They attacked as if controlled – which they were.
A nasty necromancer/alchemist created the virus that caused the plague because he wanted to take over the countryside – all because he had been laughed at by the educated elite (magical university). They didn’t think he could do it. So he did. He proved mind control [see Victorian mind-control story idea. Must be on my mind – ha!]
Many thousands died, including someone close to the investigator (Captain, Marshall, Sheriff).
The Captain/Marshall/Sheriff started out life as the son of a coal miner (from the nearby mountain), who joined the Baron’s guard to get out of mining for the rest of his life, and he was on the front lines of battling zombies. It was terrible – they didn’t die unless burned and then beheaded. The defenders were losing the war. The war broke out on many fronts all over the kingdom (thus Dead Wars).
A hero appeared, or a group of heroes, but one in particular, who could fly (some sort of cool magic) and who had fire that burned extra hot and that could not be put out with water. The fire had to be smothered with sand. The flying hero disappeared after the war(s) was won. [returned to his own dimension.]
The magical adviser that the Captain meets in the town was one of the group (secret). His job is to rehabilitate the populous by teaching them to read and write again. Most people, including the Captain, have lost most of their language skills, and so refer to complex things by the simplest means. For example, Dead Wars, instead of zombies or war against the necromancer. The magical adviser offers to teach the captain how to read and write. The captain has a thirst for learning.
The Captain also has a fear of zombies or a great hatred of zombies – or some emotional hold over from the war. Fear of fire? His job is to find any trace of zombies, so when someone dies mysteriously he has to investigate.

And so – someone in the village across the bridge from the walled town dies mysteriously. He, his dog, and his trusty sidekick go to investigate.

There is a group of people from another dimension/time that is in this time to prevent some terrible thing from happening or to clean up from something terrible that happened. The necromancer/alchemist was a part of that group, and so was the flying fire hero who defeated his armies. The magical adviser is one who volunteered to stay behind to clean up the mess. Perhaps they are ‘guardian angels’ who watch over this place/time. Perhaps they are researchers who are doing psychosocial/sociology experiments (like Star Trek Insurrection).

The magical adviser tells the Captain that there are several levels of azam (magic). Most azamers (?) (magicians) are just using slight of hand. Real azam is much rarer and takes a lot more learning and constant practice (like a martial art). When someone dies, the izam (soul) is released and a azamer can capture it. That’s how the necromancer… The Captain says who? The dead shepherd. Oh, what did you call him? necromancer – meaning an azamer who can communicate with and control the dead. The Captain looks impressed by the magical adviser’s vocabulary. As I was saying, that’s how the necromancer might have done it. He captured the plague victim’s izam.
Azam is all around [like the force]. Manipulating it is hard and takes training.