For me, the backbone of the inferno army is Nightmares and Fiends (or Overlords). Nightmares live longer if you don't immediately throw them into battle, and a clever move can scare someone. Overlords, thanks to their magic, deal good damage, and Fiends have magic resistance... they are the best unit against mages. Moreover, this unit likes to defeat numerous squads, for example, undead.
The Black Dragon's luck looks better because of its dragon breath and elemental fury!

Don't compare the dragon to Diablo in terms of damage; devils have better armor, and they can summon Cave Lords. The latter are frankly terrible because they rarely move and don't go far. Horses rarely scare anyone, and only when the computer wants them to, so it's not a tactic. And why immediately undead squads – do you mean skeletons? It's hard to reach them, especially with Fiends.
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Sorry, I just played with them, and my opinion about demons has changed for the better. After they defeated the undead, I don't consider them weak anymore. The main thing I realized is never to disdain portals; this equalizes the chances in melee combat.