The Overlord was killed by everyone together.
The quartet of Railag, Zehir, Railag, and Faydaen is also very strong. It was they who defeated the Overlord himself.
He was actually defeated in the 6th campaign...
Biara: "Do you want to take his place? Do you want to rule Shio instead of him? You're insane!"
Zana: "The Archdemon owes nothing to anyone. Especially Kha-Belekh."
The Overlord: "... I am an ancient demon. And I know how to wait. And one day I will get out of here and crush you and your minions..."
Regarding the battle between the Overlord and the quartet. I think Kha-Belekh deliberately surrendered to allow the heroes to leave with the false Isabel. Otherwise, how can we explain the fact that the huge demonic army did not participate in the battle with this quartet? If the Overlord wanted their death, I don't think they would have calmly left Shio. Moreover, Kha-Belekh achieved what he wanted - the Messiah will soon be born, and the Overlord's best agent, disguised as the queen, rules the Empire.
