Heroes VII

Hey everyone, I'm having an issue – I'm playing as the Necropolis faction, it's the second map, and I've completed all the quests except for one: Zenda, Al-Malik, and Lyudmila must not die. I've gathered the refugees, killed the lords, captured the mines, and even found the Tear of Ashi – but the mission still won't end. I went through the dialogue about the Well of Souls after finding the last refugees, and that's it; nothing else happens. There's a fog of war at the top of the map, but I can't get there; I've searched everywhere.
I'm playing on hard difficulty, and I've already cleared the entire map. I really don't want to start all over. Does anyone have any idea what the problem is?