Script: The plot is alright, but could use some improvement on the character's develepment as the missions progreses. Some dialogues could use some work (I'm not talking about the spelling mistakes, which there are but you can still understand and enjoy the game).
For example, in the prologue, the part where they scort the Necromancer's body to the crypt, that whole scene could have been with no dialogues and would be fine and the player will still understand what's going on.
Cinematics:
When you "play XX animation" on any unit, remember to put "queue stand animation". That way the character is not frozen in time after played said animation. This happened on prologue and first chapter if remember correctly.
I think it was chapt 2's endding that ended abruptly, a nice fade out would be nice.
Gameplay experience:
I like this mechanic were you get to pick your own army by using Second change. I think it's fun and new (at least for me).
Suggestion: I like the idea that you can reanimate neutral animals too! I thought this can be exploit and make secrets path were only small units can fit in to unlock them, just a thought.
It would be nice if the Main Character himself can raise the "legendary warriors". Because the first one I reanimated him with a Raccoon and the second one, with the Black Dragon (which I found out actually cool but a bit overpowered lol), without even destroying the trees.
Suggestion: since these are powerful warriors, a nice dialogue or something special would be nice, so the player can also understand that unit is a special one and a secret that there will be in most maps further on. It doesn't have to pause the game for this, maybe some small silly line will suffice.
Edit: Forgot to mention that I love you imported specific unit models when they go undead, also, they having same or sliglhy modified spells as when they lived is a big plus to the gameplay. yay!
Chap 1
I failed the second side quest when I killed one of the family peasent that came attacking me while destroying the barracks. I even didn't know he was part of a side-quests till I destroyed the barracks and tower and procceeded on.
Suggestion: If you wanna fix this I can suggest you 1) Pause the peasents till the cinematic starts, or 2) change the ownership of the family and make them treat player 2 as neutral.
Escenary:
I like what you did while creating your world. the terrain, doads, and so on looks nice. I like how, overall, you tried to keep it W3 style, kudos for that.
Sound:
You can add some sounds files when the player interacts with the ambience or when they are on the correct path. For example, 2º chapter when finding out the scepter for the side quests. It's not necessary but gives flavour to it.
As I mentioned before, I did enjoy the campaign and looks really good for it to be your first one. I'd love to see more of this story.