He defeats enemies with his main hero (either with a full army or Imps, depending on what he has), and he also uses Alastor, from whom I took Dark Magic with a mentor, and the new Leto, from whom I also plan to take it and give him Spellpower (since he has the Phoenix Cloak and 2 fire spells of level 3), the last two fight until the very end and have siege engines with a tent.
But you can't fight much with Darkness with maximum Dispel and Cursed Ground, there's no Unholy Word or Berserk/Subdue, and I haven't even found the path to the library yet, and I don't have enough Dragon Armor yet.
Since I've already acquired the Mentor and Sandro's Cloak, I don't need Markel that much, but I would really like to get Arantir, but he doesn't appear, and it seems he won't. :(