What are exactly the demonologists? Black demons, the upgraded version of the red ones?

Nope, those looks more like red necromancers.
Upgraded demons are executoniers also great unit but i count them as "time back unit" They are awesome went teloprted on the begining in the battle and timebacked at the end of the 2nd round. But hard to ress them . They are doin great also as summoned by demonologists.

Executioners are fantastic when you're up against a hard opponent with thousands of troops lvl 1-3. Just finish one of them off with his active ability and you can count on at least half of the others to be locked out for 2 turns.

Summoning Executioners with your Demonlogists is the best thing. Not only do they do a whole lot of damage due to attacking so often, but they also tend to fear and lower the initiative of enemies. Time Back on them to resurrect a whole bunch is a great combo.

I can't find any Demonologists in my game! :( But I agree that Paladins and Inquisitors are pretty much a given in any team (I take the point about Inqis losing their value later in the game though). Red Dragons are supreme if you can find them on Verona as I did. A couple of eggs can be swiped in Shettera too. I'd say at least 50% of KB players love Royal Snakes, and I'm one of them, but I have stopped using them now in late game. Archmages and Druids are also very nice. Royal Griffins too. Those are my core units.

I found five demonologists in my latest game on Bolo and I'm hoping there are more somewhere. Not very hopeful, though...

I didn't find any demonologists in my first game as a paladin but my mage found 140 in Verona - Sweet! That barbarian encampment on the west coast always seems to give me good stuff.

Demonologists are great but I also find Trolls and Ogres are awesome if you can find a good amount of them. Ogre's skill to steal AP is very nice as you can pretty much charge into the mobs with 1 turns. With Ogre Boots and Club you can even get better.

The combo I *wish* worked: Casting Doom on your own Vampire Lords.

Alas.

NIce ida :p