The mortuary arts are delicate and precise. Some brutes hacked away at corpses like a butcher hacking meat, and then had the gall to proclaim their "findings" to be legitimate; Nergal had met a fair few in his travels, both as colleagues and rivals. Not so with him: his work was always precise and meticulous, all his cuts careful, and all his observations thoroughly noted in the mass of notes on the table next to him. As he cut into the specimen's lungs, a terrible stench arose that threatened to overwhelm even his own dulled senses. Clearly, the child's body had been buried for too long to be of much use, but he had had little choice in the matter, all the fresher graves near the town walls being too risky to excavate.
He currently worked in the cellar of old Yorm, the aged local for whom he was acting as physician. In return for caring for his host's myriad of health issues, he was permitted to peruse his research unmolested in the dark cellar, far enough underground for noise and stench to be masked from the surface. To be sure, Yorm didn't know what exactly his "research" was, and if he did he would likely throw Nergal out, skilled medicine or no. He would likely be chased out of the town entirely if his secret was known. Most scholars were irrationally close minded about the learning of the dead, let alone the superstitious and ignorant townsfolk of Combedean. It would be no use to reason that knowledge of the dead is required to treat the living; already, he knew that suspicion was drawing close to himself, his grave robbing having come to light recently. He would have to move towns soon enough if he intended to stay away from the gallows.
When he was satisfied he had learned everything that could be learned from the corpse - as little as that was - he gathered his notes and sketches and swept the rotten bits of flesh that were left into a large pot. He made a mental note to empty the rapidly filling pot that night - he knew Yorm would be visiting a friend, leaving him free to sneak it out to dispose of in the local river. He washed his hands thoroughly, as was his habit, gathered his things, and climbed up the stairs leading to the rest of the house. He swung the thick wood door closed as he passed through and locked it. To be safe, he pulled a large stone the size out of a pouch in his robe. The Black Shard pulsated as he whispered the words of power that installed an Alarm spell on the door. Should any intruder mess around with the door, he would know it immediately.
Satisfied, he left the rustic manse on the outskirts and made his way into the heart of the town. He needed to purchase some salves required for Yorm's gout, much as he disliked the bright daylight and the crowds. He was vaguely aware of some kind of festival coming up, though he cared little for such things himself. As he passed near the Church of Kaher he became aware of a great deal of noise. As he approached the site of worship he found a mob gathering in front of it, cheering on as two bands shouted at each other. Why, he could not guess; the proclivities of the townsfolk were both barbaric and nonsensical to him.
He was just about to move on when a very large and very imposing dragonborn pushed through the crowd and interrupted the commotion. The creature intrigued him; he was equipped as if for war, but did not appear as the usual drunken guards did. Nergal remained standing in the back of the crowd, watching. For reasons he could not quite say, his attention was piqued.
It's less that he can arrest the Necromancer and more like he just doesn't want to work with too many practitioners of magic that are either uncontrollable or malicious in nature. Plus, I have more characters in queue, anyway.
Just to make it clear, my necro isn't malicious, he just sees necromancy as a tool to be used. But if you insist.
To any possible joiners: We could use a rogue or charisma leader for the party still.