Demon invasion.

I somehow missed this comment. I understand that a lot of time has passed. In the end, was the problem resolved or did it remain?

Added 4 minutes ago

I don't remember the map very well anymore. It is desirable to complete the lower part as quickly as possible. I have a few questions:
1. Does the hero have his special spells? (hell wave, devilish strike, etc., as well as the call of Urgash)?
2. Are there any modifications installed (EWA, for example) or third-party maps that are mods?
3. What happened before the transfer to the surface? (did you attack the last demon or did he attack you? This may be important).

Yes, the spells and master ability are definitely present; without them, I think it would be impossible to get past the first stack (or even that). As for third-party content, the first campaign about the league is installed, as well as campaigns from previous parts of Heroes 5.

The transfer occurred after attacking an enemy Castle where the last demon was holed up. And in general, the transfer worked as it should (based on the comments in the thread): the stats dropped to 1 1 1 5, the hero had nothing left except 3 spells and remnants of the army, but there were too many powerful stacks around. A hundred Cerberuses/Furies/Scouts are generally invulnerable with magic skill at 1, because 60 damage from the fire wave is really insignificant. There were also 3 hundred zombies, which in theory could be kited, but spending 40 minutes on such a thing is already too much for some.