Hi. I played all the chapters. Regarding the story, I quite like the concept but if you're following original frozen throne lore then it would make more sense just summoning Kil'jaeden instead of Sargeras just like that out of nowhere. If you're not then that's fine. A little backstory on this Dreadlord and where he came from wouldn't hurt a little but I think that's something you might touch on later when you're done improving other things.
Now regarding gameplay:
I won't mention grammar as that's something you can take your time with.
The terrain doesn't bother me much as I'm not an expert at it but other people who decide to comment here will certainly say something about it.
The AI is not that good, for all of the maps. I think you can look around for AI scripts from other maps and try see if they match with the dynamic of the maps since most of the enemies in this campaign are all default units/races.
Some triggers need improvements, couple examples: On the mission that you have to find the demon gate, not only can ANY unit enter the teleport and trigger the "need to kill all enemies" message (or go into victory cutscene when you killed everyone) but it also spams that message when you enter and leave that teleport repeatedly. It would make more sense for that trigger to only apply to the Dreadlord and you could also just disable the portal until all the enemies are destroyed and then enable it.
Next mission, the orc clans. I think AI is OK here. The issues here are: the pitlord's ultimate. 4 infernals makes it too easy on top of the fact that you can stack good items on him and get cleaving attack level 3 which is a very good ability therefore he's kinda OP because of it. The other issue. dreadlord's ultimate: Charm. If you possess a wisp or peon, you can build any unit, any hero from that race. AND on top of that, you can literally make infinite heroes, there is no hero limit since that trigger was probably deleted. You should disable orc and night elf units/buildings for the player. (I didn't try using banshee possession on other missions but I'm assuming the results are the same)
The Nordrassil mission: there's no indication whatsoever as to when the mission ends or how long you have to survive. It's just peons chopping the big tree and that's it. I stayed there for 20 minutes and nothing happened, so I just quit and moved on the next one.
Overall, needs some cinematic/trigger adjustments and maybe map changes so they don't feel too empty. Also try adding some more custom models to both the player and the enemies (like the orc heroes gathering). You also should just go full demon race with custom model workers, warriors etc.