Revived

I guess I'll have to change the quest – to study dark spells instead of light ones, and generally switch from light to dark, whatever.

And that's good! Because a drug addict's nature is dark – there's no point in forcing him to study light. If Hottabych were on the map, I wouldn't hesitate to switch from light to anything.
It's a pity that Stefan can't be given the Cloak of Darkness, otherwise he would become a truly terrifying monster.

P.S. The map is great and the plot is interesting – thank you!

Added after 19 hours and 3 minutes
I captured the outpost guarding the entrance to the green territory, defeated Dunkin, and headed towards the city. While I was walking towards it, Dunkin reappeared and almost caught up with me near the city. I decided to experiment – what to do first – kill Dunkin or capture the city – and realized that the game has a continuation, but only with the second option – first capture the city, and then kill Dunkin; otherwise, it turns out as described by elizbar1967.
P.S. By the way, it's funny – I captured the castle with Stefan, but Kolka got the sword. :)
After capturing the green castle and defeating Dunkin, Stefan urgently returned home, and with the help of Iris, he managed to transfer his troops to Kolka before the arrival of the third orange "G..." (I forgot his last name), although this wasn't very important – Kolka had already managed to gather an army of six dozen wampires there (most of which were made of griffins and wolves) + skeletons and zombies from the castle and creature generators. As a result, the redhead showed up and was brutally defeated – he was crushed with almost no losses (a dozen skeletons and a couple of ghosts don't count).
P.P.S. Then I put Stefan in charge of guarding the city with all the zombies, and Kolka went to deal with the hydras, got the boots, and when a new orange "G..." appeared again, Kolka met him again, but this time he didn't reach the castle – they met at the entrance to the portal, and this time it all came down to beating up the babies – he dealt with his army with vampires alone, without any losses, and Kolka's level is now almost at its maximum.
And the red demons are quite peaceful if you block their exit from the teleport with a powerful hero (Kolka) – they just dig around on their island and don't bother me, and their danger level is always "very low."