The most useful monster.

I enjoy playing through campaigns when they are interesting. The last one I played was poorly designed. In the first map, I could easily level up. In the following maps, even at the beginning of the game, when I only had one hero, the level of my army was ridiculously high (in the Thieves' Guild). I just walked around and cleared the map, while the AI took a long time to make their moves. I wasted so much time.