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I completed the campaign with this hero, and I must say it's one of the best I've played in this game.
The downsides: The second, third, and fourth missions are too easy.
In the second mission, the only challenge is at the beginning when an Orc and a Demon rush you on the third or second week. If you survive that, you've practically completed the mission.
The third mission only requires some tactical thinking at the beginning to combine your heroes and defeat the first enemies. After that, I simply stacked 100 of Lucretia's Vampires in the first month using Necromancy and used them to capture all the Mage Castles. Once I killed everyone, I leveled up Markel on neutral creatures to level 28, and by the fourth mission, I had two overpowered heroes with a ton of dark energy. One of them, Markel, didn't even need an army; his Phoenix destroyed everything in its path. The other stacked a large number of Vampires, and by the end of the second month, I had 300 Vampires that could solo the last Castle in the fourth mission.
In the fifth mission, I decided to make things more challenging for myself and play Sandro using only Dark magic, without summoning. This is where I really had to work hard. I killed everyone without any losses until I reached the hero guarding the Spirit. That's where I lost my entire army, but luckily, I had 70 Mummies left, which I stacked in groups of 10. Then, using Resurrection, I attacked the Dragon with auto-attack for about 30 minutes. I destroyed its defense using Destruction and Angels through Subjugation, trying to deal some damage. Overall, it would have been much easier with summoning.
Thank you for creating such a wonderful campaign.