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Well, I guess I'll share my opinion and rank the heroes in order of preference, with comments explaining why. There will be 7 entries because I don't play as Delab at all, although in 3.1 she's not that overpowered, she's just a good "mechanic." But still, having machines from the start and the bad reputation of being a "noob choice" is too much.
Recently, I've been trying out Jezebeth because I simply want to play as a creature specialist, but I'll be objective – the starting skills don't allow me to place her high in the ranking.
So, let's talk about the demons.
1. Nibros.
The hero has excellent starting skills – Attack + Tactics, allowing him to clear out archers and spellcasters in reasonable numbers with Cerberuses, losing 1-4 dogs. A single point in Luck sometimes helps a lot; you can counter the tactics of elves and position yourself for an attack.
2. Grok.
He's equal to Nibros; I put him in second place solely because, on some maps – I emphasize, on some – Logistics is sometimes unnecessary; it's better to focus on Attack, Defense, and Magic right away. The 6% bonus to mana, plus further bonuses for levels, is very good, and Teleport often helps to clear out neutrals without losses.
3. Marbas.
He's a good hero; he has built-in Defense, although with an unclear Reflection, which prevents taking Endurance + Block or Mass Skin/Evasion. He's in third place because he's not as good against power-buffing heroes (elves, orcs, humans), and on standard maps, you can often get the Staff of Sar-Iss, which is unpleasant when the hero's specialty is completely nullified by a single artifact.
4. Nimue.
Along with Delab, many beginners advocate for this hero because "having a lot of gates is cool." In practice, he's situational, even more so than Marbas. He's good against Necromancers, Knights, and Dwarves, more or less against Orcs and Mages, but not as good against Tier 3 heroes, and useless against Elves. Luck is decent, although not on my list of top priorities; Magic Expertise is more of a yes, sometimes no.
In my opinion, the good heroes end at this fourth position; after that, they're worse.
5. Jezebeth.
Buffing demonesses is good, but the starting spellcasting spoils the picture. It takes a long time to develop, and Wisdom only helps with Phantom (although if you manage to get the spell, Jezebeth becomes incredibly powerful, but it's too random). I use her because I like creature specialists; in my opinion, she's the best for a long game.
6. Grole.
I used to play him often, but now I've come to the conclusion that buffing dogs isn't worth it. Like the Furies of Tier 3, these damage dealers die very quickly, and Grole's specialty almost doesn't affect their survivability. The starting skill – Developed Chaos – is only suitable in the early game to counter mechanics and hunters. Developing towards Implosion after cutting Hellfire doesn't pay off. Weak.
7. Alastor.
"The dark horse" of the demons. In principle, if you get Darkness early, he can outperform even Delab and Grok combined in clearing out areas (I set my personal record on the Dragon Pass with him – by the end of the third week, I cleared out my own territory and the Dragon Pass completely). The problem is that sometimes Darkness isn't offered, and by level 15, the starting Spellcasting is completely useless. Also, I personally don't like predictable heroes – when playing against Alastor, you know for sure that he'll have Darkness, and not Light, for example, and you prepare for it. Also, in the case of this hero, no other skills are enjoyable until you get Darkness – neither Logistics nor Attack; you just wait for Darkness and get nervous. Not for me.
Delab is "beyond Good and Evil." In rush games, if she's not banned, she's still the best. In long games, she's the worst choice. This hero doesn't exist for me; she's permanently banned, so I didn't include her in the list.