In the depths of Hell.

I wouldn't include Destruction magic in this list if you're choosing Dark magic for your hero. 2 magic skills is too much. Attack, Luck, Leadership, Might, Logistics. But this is just one of the options where the focus is entirely on the army. You can also develop the hero, focusing on him, and then a completely different set of skills will be used.

Added 5 minutes later
Throughout the entire playthrough, I used Destruction magic about 3 times. All the mana that remained with the hero after the demons' turn was used for teleports, creating phantoms, mass slows, vampirism, and so on. In the previous map (The Outcast's Path), Destruction magic was more preferable.