Come to think of it, Miosguinn's behavior is rather strange. And the fact that the they were attacked by a bunch of
giant spiders back in the library should be brought up again, given the recent development:
Amena frowns, "I... we were in the stacks and he saw a tome he wanted... he grabbed it... it was black and had some sort of image on it but..." it is clear that she is not upset but rather is having trouble remembering what she saw or perhaps she simply does not wish to tell you.
She shakes her head and gives you a worried smile, "Sorry Derryth, I do not know. He grabbed a book and put it in his pack and while I was watching him we were attacked by a bunch of spiders. I should have been paying closer attention. He fought... yes he fought them off and then Ithapi arrived and brought me to Sir Gareth and the rest you know. If it had not been for Miosguinn I would probably be dead," she shudders.
So the spiders attacked while Amena watched him, and she isn't sure what happened, but she thinks he chased them off... or did he? Could it be that Miosguinn is one of the cultists?
Now let's try and remember how we came to adventuring together:
It seems you will not be able to meet with the Circle today, many of its members are out of the city but Miosguinn himself is quite interested in your proposal. It seems that the Smiths of Muirthemne are a topic near and dear to his heart and he agrees to accompany you with his apprentice Isolde. You assume that the bookish girl popping in and out of the room and giving you dirty glances must be her.
If he is one of them, that would explain his interest as well as the hatred of his apprentice, given the attitude the cultists have for the ones who try and get the artefacts of the Smiths for themselves.
Ah, well. No point in dwelling on that now.
If you want a real plan on how to proceed from here, how about this one? You finish the battle, either by helping Thais (I don't like the perspective of her being one-shotted) or Tyrvard (killing the leadership might be best, but I am hesitant to lose more people). Then you treat Amena and Ithapi. Then you try and find Miosguinn, Isolde, Garreth and Brigit (now where the hell are they?). Then you get Nine to help you track down the cultists that took Serpent. They have to be somewhere nearby, and if there are mages among them, Nine might be able to feel their presence. If she tries anything, threaten to gag her and to throw her in the deepest hole you can find. If Miosguinn is not actually the one who got you into this mess, have him cover your whole group. Have Nine give him pointers if needed. Tell her that if you are spotted, Tyrvard will take his time to bash her skull first. Then try to locate the mage, give Tyrvard the stones, all five of them (take the one from Miosguinn if needed), and send him after her. If the stones work like they are supposed to, the mage will be powerless. Kill her, grab Serpent, and get the hell out of there on the double.
I think it is too early to write one of our teammates off.