Walkthrough for the "Tribes of the East" expansion.

Hello everyone! Can someone help me with the first mission of the last campaign, specifically the one involving Zephira – "Deeds and Paths of Darkness"? I go down into the dungeon, but the mission to find out something about Isabel fails. But that's understandable; it's designed that way. Then, I need to clear this dungeon of the Dark clan that's present. But how? They have four fully developed castles there, plus a decent-sized group of units and enough troops; they're too strong for me. So, how should I proceed? :confused: Thanks to everyone!