Auto-translated
Arengar, even in the Campaigns, I maxed out Zehir's Light, Summoning, and Empathy skills, along with Machine Loyalty. I paid a huge sum to the mentor for Leadership. Zehir soared through the ranks like a phoenix. No enemy could cast a single spell because Zehir managed to cast a counterspell and cast spells for himself. Fortunately, there were no Necromancers with Spell Power among the enemies. But even if it's not possible to get Leadership on the map without a mentor, a mage with Light and Summoning is always combat-ready, even without Empathy and Spell Power. The main thing is, there's no risk.
I voted for the Call of Urgash. Not because it's the best, but because, in my opinion, it most significantly enhances the hero, if we exclude a mage without maxed-out magic skills. And most importantly, my soul rejoices when I get any ability that leads to Spell Power. I greatly appreciated Cursed Land during the Campaigns. It completely prevents enemies from summoning Phantoms. One stupid AI kept summoning them throughout the battle, which is equivalent to skipping turns. Even a sober person can fail once per battle with a Phantom if they don't know the enemy's build. Well, when playing with friends, we usually drink beer or something stronger, or both. And skipping a turn with a hero can be fatal.
In general, I don't know how to play as Demons. I fall behind in development from the very beginning. I didn't take Deleb even before version 3.0, and now even more so. And then it's not always possible to catch up. Maybe because I haven't played as them much. When playing with friends, I play as the race I'm playing the Campaign with. And for Demons in the vanilla game and two expansions, there's only 1 Campaign and half of the bonus mission "Dark Messiah".