After clearing necropolis hero-champion in last campaign I do not get access to the next cave enterance?

So I did a blitz with my necro hero taking frogs castles within two weeks and went forward killing orcs and opened the first dark barrier. Then I cleared humans and disabled the second dark barrier and accessed necromants part. When I captured all their castles, I got to their main hero who guards a closed (glowing) cave entrance. I killed it, and the entrance is still blocked. Anyone have an idea if I missed something?

My main hero is blood necro mage. I cleared almost all monsters in every corner of the map. Except 1 group of Nagas near the starting location which I doubt makes the entrance locked.

Solution 1:

In order to make sure that all the prerequisites have been met and all forts captured on each mission, you can check the link below:

https://guides.gamepressure.com/mightandmagicheroesvi/

It has all the hints, maps and everything you need for each mission