"The Will of Ashan" - "Heart of Darkness" [2nd campaign, 5th mission]

Yes, but if you kill the assassins, they won't kill the succubus. And if you can't kill her, you can't destroy the portal!... And now I want to free the captive... She has a really great army)) And it would be a shame to turn it into undead)).
So, what should I do?... I can't figure it out((( Either kill the assassins, but I don't know what to do with the portal then... Or they kill the succubus, but I'm left without the reward)...? Please, help me))