The most challenging campaign.

I encountered some kind of weird bug in the first mission of the Necromancer campaign. You have to sneak past patrols, and those damned Indians on elephants just stood in the middle of the passage and wouldn't move! You can't even attack them – there won't be a battle, you'll lose immediately, so I restarted the mission three or four times.
I also didn't like the fourth mission for Agrael, where you have to capture a port city, and at the beginning of the mission, the army starts to desert. There's nothing particularly difficult about it, but the fact that they deserted just annoyed me!