In the depths of Hell.

+4 to Attack and Magic Power is certainly nice, but it doesn't even come close to the Staff. Those 20% provided such a huge advantage, it's incredible. I've been playing the third map with this hero, and this artifact allows him to obliterate almost any army. In one turn, no one can reach you, except for black and green dragons (-1 to speed per turn) and -20% to initiative. We also need to consider the hero's passive ability, plus -40% from Dark Magic and +30% speed buff from the Attack skill. And I was destroying everything that moved...

In general, I was doing well storming castles until I encountered Daleb (I think you remember, but just to clarify, it's a demoness with an explosive ballista)! She simply obliterated 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 was already out of Miggers and my Lizards were battered. And if I tried to destroy 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 my army.
As for the Devil, it's my own fault; I just dragged out the game, and he destroyed me with his army. The Imps were annoying at first, but my initiative saved me; I blocked them with the hero before they could do anything. And it wasn't his magic resistance that killed me; more than half of my spells were useless, and I could only cast control on the Imps and destroy half of his army with them, but then Archdevils destroyed them, and no other spell worked, I managed to cast a few more into the void). And it happened again and again; if even one control spell worked, that was a success.