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A simple walkthrough for this mission (assuming you have Astral Gates in your spellbook and can summon a Phoenix).
Head towards the garrison, bypass it. Capture the first Castle. Immediately go right to garrison 2. Bypass it as well. Proceed to the middle left Castle. Along the way, defeat the computer hero. Capture the second Castle. Then, go to the upper right Castle; I encountered another computer hero along the way, which I defeated without much trouble. The third Castle is straightforward; my army was weaker, but the Phoenix summoning spell gave the computer no chance. Summon the Phoenix, send it behind the Castle walls towards the archers, and it will eliminate them all. After that, it's quite simple. Then, go down, capture the middle right and lower right Castles. Of course, after capturing the Castles, replenish your army a bit. In the first Castles, I only recruited archers because I had problems with resources and couldn't build much in the captured Castles; the trading post near the first captured Castle helped. By the way, the upper right Castle was almost fully built by the computer, so thanks to it. There's a stone mine at the lower right Castle.
Once the computer player was gone, I went to the Dungeon. To make it easier to navigate with a decent army, I cleared both garrisons and recruited another hero from the tavern near the Dungeon (I had to clear the garrisons because the newly recruited hero didn't have Astral Gates in his spellbook; it takes too long to explore the Dungeon with just one hero). By the way, I couldn't find Godric and decided to explore the lower left corner, as I hadn't been there before due to focusing on leveling up Zehir. I went there as soon as I gathered everything on the map, defeated almost everyone, and received a message that I had reached the maximum development level for this stage. Only then did I go to see what was happening behind the lower garrison; I thought the prison with Godric was there. After bypassing the lower garrison, I reached Markel. I was offered to fight him, which I couldn't refuse. However, since Markel had been significantly leveled up in previous missions, he defeated me, but only once. On the second attempt, I realized that the trick was in his duplicates, which appear and shouldn't be focused on; you should only target the main troops. Another difficulty was that he had the spells "Dominate," "Berserk," "Summon Phoenix," and "Create Phantom"; I only had "Summon Phoenix" and "Create Phantom," but I still defeated him, after which I was kicked out of the game. I decided it was a bug and tried again, as I always save the game during gameplay. After the fifth attempt, I realized that I needed to change something and started looking for Godric or Faidaen, not knowing where or how they would appear, which annoyed me. I went into the Dungeon and, surprisingly, quickly found Godric's prison. After that, I saw one cutscene, and then another. Faidaen appeared in front of the lower garrison. I defeated this garrison with him and went to Markel. I reached him and waited there for Godric and Zehir. On the way to Markel, I bought more troops in the Castles and arrived with a decent army. Then, I redistributed the troops, planning for the first battle to be difficult, and the next two easier. The first battle was with Zehir, then Godric, and finally Faidaen. By the way, a tip: before the battles, have all the heroes approach each other, and they will learn spells from each other. For example, Godric and Faidaen learned "Summon Phoenix," and Godric learned "Teleport to Castle" (which will be useful in the next level). Also, I noticed that if you refuse the first offer to fight Markel and attack his Castle instead, he somehow fights less aggressively and doesn't use "Dominate" and "Berserk" as much as he did before.
The difficulty level is Veteran; I haven't tried Hero difficulty. I completed all levels on Veteran. So, good luck to everyone! I'll try to finish the fourth mission of this campaign; I haven't completed it yet.