How exactly do they work?

Currently I'm playing the "Half-Dead" campaign, and managed to make the campaign hero a Demonologist (combining the Death and Nature magic character skills).

According to the game's wiki, for example Scouting and Nature magic turn a character into Bard.

Does that mean, that if I get Scouting in the future, it would turn the character into Bard, costing him the Demonologist's bonus? I know advanced classes can change (in the Preserve campaign, my character changed from Warden to Archmage).

So, how exactly does that work?

Your hero's switch to the Bard class will happen when your combined Scouting skill exceeds your combined Death Magic skill. For example, if you have Advanced (level 2) Death Magic and Basic (level 1) Occultism, this results in a total value of 3 skill points in the Death Magic tree. If you then train your Scouting to Advanced (level 2) and Pathfinding to Advanced (level 2), this gives a total of 4 skill points in the Scouting tree and your hero will convert from a Demonologist to a Bard.

Note that if your hero has an equal level of skill in each, they will retain the advanced class that they first reached. It is only upon exceeding the first skill tree that the hero's class will be promoted.

Generalizing from Rmeakins' information slightly: a hero's class is determined by which two skills have the most "points", where one point is one level in that skill or any sub-skill associated with that skill. So Basic Occultism is worth 1 point, while Advanced Occultism is worth 2 points. You need at least 2 points in a skill for it to count toward an advanced class (having just the base skill as Basic with no supporting skills won't move you to an advanced class).

You can have a max of 20 points in a given skill (5 for the base skill, +5 for each of the three sub-skills). Once you max out two skills, your class is set since no other skill combination can exceed that max.

I believe Archmage is the only exception, since it requires 3 magic skills. I might be misremembering because it has been so long, but I seem to recall that once you become an Archmage, you'll never switch out of it.

Great, thanks. :)

Which other skills would you suggest for the Demonologist, btw?

I ended up with Combat as a third one so far, though I might yet restart the campaign, if I'd want a different set. Thinking about Nobility (for a bit of resources), Tactics, and Scouting (mostly for Pathfinding, though being a sneaky summoner won't hurt, either). Or I might just ignore Nobility and try to get the other two, extra toughness from Combat is not something to scoff at, I suppose.

For the base campaigns, Combat was always a No Brainer to take and fully max. Lots of defense, double attacks with First Strike, double ranged attacks with First Strike, and eventual magic immunity. When you factor in all the stat boosts you tend to find on the map, your primary hero becomes a soloing monster.

Take the magic skill aligned with your campaign (ex. Death, for the Half-Dead campaign) since you'll be seeing plenty of towns to collect those spells.

I didn't bother with Tactics because the primary hero eventually becomes more effective on his own because most creatures slow him down on the main map. [Edit: If you aren't letting your main hero travel solo, then Tactics is a great choice for all the creature boosts. I don't think it helps any heroes in the group, though.] [Edit 2: On reflection, also consider Tactics if you do a lot of summoning in battle.]

Likewise, I wouldn't bother with Nobility for the main hero because it doesn't help your solo hero kill things. Buying level 1 heroes with Nobility and popping them in a town to increase production (and for their Estates skill) was usually a goal in each map, though.

Scouting is pretty meh, since Seamanship and Stealth mostly don't matter for your primary hero. Pathfinding is nice, though. Tawni can make good use of it in the Pirate's Daughter campaign, but she starts with it. [Edit: If you have high Stealth, you have to actively attack the neutral guards on the map once they stop responding to you, which does burn some movement and is some extra clicks. Generally you want your main hero (as your strongest force) to clear the guards out of the way so other stacks or caravans can make their way to places.]

Basically, I'd generally take Combat and as many magic skills as I could (with preference for the ones friendly to my current campaign's alignment). Even if you don't use many of the other spells, their secondary magic booster gives you more spell points and spell point regeneration, and having a 20% boost to spell casting across the board (Archmage) is also handy.

Just give all stat boosts to your campaign hero and carry around a few Potions of Immortality as soon as you can afford them, and you're golden by about halfway through the second map of each campaign.

[Edit: Since I have my hero traveling solo, I avoid Nature's Summoning skill like the plague. The extra management dealing with the units showing up each turn is tedious work to slog through before my hero can get back to the slaughter.]

Playing a summoner-type character in this campaign, I think Tactics might be handy for the summoned units.

I agree with you about Nature's Summoning though, while it kind of worked in the Preserve campaign (having the elementals soak some attacks, until I get the unicorns/phoenixes/other summons rolling, especially as I didn't have Combat there), I'm going to try and avoid it tihs time.

So, I'll probably go with Tactics and Scouting, leaving Nobility to other heroes, if I manage to get them (I think not all city types can recruit Order heroes, who typically have it).

Combat and Death are obvious choices for Gauldoth. I think Nature is pushed for story reasons, and the demonology makes it not bad. I'd personally go for Chaos (damage) or Order (disables) or both, but Tactics works too. That said, if you like Scouting then go for it. The extra sight range and pathfinding are nice, but make sure to avoid Seafaring for Gauldoth's campaign.

You might be able to shove the nature summons through the Undead Transformer to convert them, if you're willing to deal with the extra unit management that entails.

As far as recruiting heroes: a tavern in a city will offer heroes of the city's alignment, plus heroes of the two friendly alignments (one section away in the circle). So Death cities will offer Death heroes as well as Order and Chaos heroes. Might cities will offer Might heroes and give access to the might oriented heroes of all five other alignments (but not the magic oriented heroes). Therefore, only Nature and Chaos towns can't purchase a might oriented Order hero. Be aware that heroes can be purchased at a discount if their alignment matches the alignment of the purchasing city (so buy Order heroes at an Order city if it isn't too far out of the way).

I seem to recall a free standing tavern on the adventure map might let you purchase any alignment of hero, but those are very rare in the base campaigns. I think adventure map taverns don't give any discounts, either.

Combat and... who cares.

Seriously, the flaw with HoMM4 is that Combat is so unbalanced, that for single player campaigns, except perhaps the first level of each campaign where the hero is still weak, nothing else matters and the game turns from a strategy into a pseudo-RPG.

From the second game level (I mean the second level in the campaign, not hero's level), hero will generally travel alone together with one stack of the fastest units to increase his movement, and as much resurrection potions as he can carry, and just mop up the map. The extra stack of units will just stay behind while hero fights, though there might be some fights, against boss heroes or a nasty combination of wandering creatures that for example might insta-kill him bypassing resurrection, where your escort units might be actually needed.

For the remaining skills, the extra movement from logistics may be good and you can't go wrong with the spell schools from your town, or even the enemy towns. Tactics can be good for your extra stack and Nobility for the resources/units to defend your city in non-linear map where the enemy might have paths or one way portals to attack your city while you're mopping the map. But really: it doesn't matter.

Take Combat (with Resistance having least priority), then Main spell school. Then movement, or take whatever strikes your fancy since it doesn't really matter. For my choices after Combat sometimes I'd take something else just to try a different Advanced class or have something I never tried before. If you have Combat you can't lose.