Warlock – “Raelag’s Offer” [Campaign 4, Mission 5]

Please, can anyone tell me – I managed to capture all three castles after a lot of effort, and I killed everyone except Groll – I couldn’t kill him – there were too many people (53 archdevils alone, 74 leveled-up lords). I finished the mission by running away from Groll – but a question arose. I was running around for more than a week, and he didn’t have a single city left. Why didn’t he die?