Finally, I found the time to finish this mission. It turned out to be too late to kill the blue army – you suffer significant losses, and then, even with three castles, it takes more time to build up new forces for the main battle than if you were building up from two castles without a fight.
Kabelech is, of course, ridiculously overpowered; the biggest surprise was the phoenixes (I almost freaked out when they appeared after the dragons were destroyed), but I still managed to defeat him (I'm attaching the replay of the battle). After this victory, I was quite puzzled that Sigrif stopped being my hero and switched to the orange side – I thought it was some kind of bug. I thought that, on the contrary, after defeating Kabelech, all the forces and heroes of the blue army would become mine, and I would have to urgently mobilize them, bring them in, and prepare Sigrif for the main battle against the incredibly powerful Duncan and Faydaen (level 40 heroes with thousands of level 7 creatures), hoping to weaken their forces first with small forces of auxiliary heroes using "hypnosis" and "berserk" (I specifically prepared another demon and two undead in addition to the demons I already had for this purpose).
But then I discovered that if I continue playing, Duncan and Faydaen, who are quite weak, will appear in the Duchy of the Deer on the next turn. Moreover, these guys didn't even want to come out from behind the green gate until I retreated with the demon that was guarding the exit from the Duchy of the Deer from unwanted guests and opened the way to the castles for them; only after that did they come out of the gate. These guys turned out to be weaklings compared to the strength of my demon – I killed them both in one turn without much effort and without even thinking about losses, although I could have done without them (I'm attaching the battle with Duncan, but I simply forgot to save the battle with the elf – it was very uninteresting and easy).