-saske-
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Which heroes do you send to the Circles of Chaos? You mentioned using an extra hero to reach the Spirit, so you should send Ornella and three more Necromancers to the Circles, who can be hired at the tavern. Personally, I sent the Assassins to the Demoness, and I had no problems with the Circles. Try it, good luck!
I can only hire 2 heroes from the Necropolis, so I end up with:
Arantir, Ornella, Kaspar, Naadir + there's also a Knight, Valeria, who's stuck in the Dungeon, and I can't hire anyone else.
I've already tried all the combinations - nothing works.