In the depths of Hell.

I had to make it so that only one upgrade would be applied, because everyone has different tastes, and it's impossible to tailor it to everyone, so I decided to do it for the 3rd upgrade. Personally, I didn't have any particular problems storming castles without archers; the special ability played its role. But why couldn't I kill the Devil? Was it because the Imps stole all the mana, or was there another reason?
By the way, the Staff of the Underworld should have been given to the altar, and the hero would have been left with only the Scepter of the Underworld – the artifact itself is powerful, +4 to Attack and Magic Power. It probably didn't need to be removed. Thank you for the comment and detailed description.

+4 to Attack and Magic Power is certainly nice, but it's nowhere near as good as the Staff. Those 20% gave such an advantage that there are no words to describe it. I'm playing my third map with this hero, and this artifact allows him to defeat almost any army. In one turn, no one will reach you, except for black and green dragons (-1 to speed per turn) and -20% to initiative. We also take into account the hero's passive ability, plus -40% from Dark Magic and +30% speed buff from the Attack skill. And I was defeating everything that moved...

In general, I was storming castles normally until I ran into Deleb (I think you remember, but I'll clarify, it's a demoness with an explosive ballista)! She simply destroyed me with her ballista. And by the time she came out of the castle and opened the gates for me, 3-4 turns had passed, and I no longer had any Migs left, and my Lizards were damaged. And if I destroyed the ballista with magic, the army would go wild (Succubi and jumping demons – I can't even remember their names, level 2 of the Abyss). I could kill her, but I would lose half of my army.
And with the Devil, it's my own fault; I just dragged out the game, and he defeated me with his army. The Imps were annoying at first, but their initiative saved me; I blocked them with the hero before they could do anything. And it wouldn't have been a problem if it weren't for his magic resistance, which made more than half of my spells useless. All I could do was cast control on the Imps and use them to destroy half of his army, but then the Archdevils destroyed them, and no other spell of mine worked, I managed to cast it into the void about 4 more times. And so it went on, again and again; if even one control spell worked, that was a success