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In general, I decided to give it a try (difficulty level: Hero). I'll share my impressions, comments, and suggestions.
1) Hero. I would still make him a knight with built-in necromancy (MakeHeroNecromancer, immediately at a skilled/master level). Like Ornella in the first mission of the ToE Campaign. Why?
Specialization - Plate Armor. And they need morale. They don't often go with a necromancer hero. The same goes for peasants/archers. + With a human build focused on attack/defense, they will hit much harder. + This will free up a skill slot.
If we still want to keep him in the necromancer class, at least give him Leadership.
P.S. It's better to create the hero yourself rather than using dialogue-based ones. This is done similarly to buildings.
2) Geralt's path isn't highlighted. Is that how it's supposed to be?
3) Fog. It's too dense. It's impossible to see the map properly from a normal height; everything is hidden. It literally strains your eyes.
4) Upgrades/Witch's Huts. There are definitely not enough of them.
5) Spells. There are simply none. I was only able to fight without losses thanks to the Fire Warriors, and even then, I got stuck in Waterway (screenshots attached, day 1, week 5, month 4). Temples are needed. Maybe give him a Phoenix, or the Crystal of Secrecy. Darkness is probably too cliché and overpowered. But creating a kind of "Death Knight" with serious attack/defense + summoning would be very appealing.
6) Assassin's Bureau.
a) The texture (Icon128) is just a wrapper for the artifact market; it looks meaningless. I also have a similar building on my map, which is a coincidence. I'm going to guess that you were looking for a texture in the thieves' guild/tavern. That's the texture you should use: Texture:UI\Tavern\ThievesIcon.
b) For some reason, only Assassins are recruited, even though the description says all 3 types.
c) It would be better to make it a DUNGEON_MILITARY_POST type. Then the previous problem would be solved.
If the building is created manually, you can assign any type to it – it won't affect existing things in the game.
7) Artifacts. The Traveler's Boots are essential. Without them, the game drags on, and Waterway accumulates a huge garrison.
8) Dwarf ally. He could at least help somehow. Maybe I haven't played enough to reach the point where he joins the game... but still.
9) Demons are weak. They could at least be a little stronger...
10) There probably could be more creatures on the map.
But overall, a knight with necromancy is very cool. Keep it up!