Here is what I did:
Rise of the Runelords spoilers:
Anyway, that's what I did, maybe not the most awesome, but it may give you an idea or two. My idea was that Nualia and her over all ideal didn't really fit with the whole greed thing too well. She was much more the "watch the world burn" type imo. So Nualia escaped without a trace.... What I did south her is to have the letter to Aldern from Xanesha in the second book include something hinting at a new ally (Nualia of course) I included enough description that they were fairly sure of who it was. I also included something about her greed and lust for vengeance eventually providing a great soul for her master once her usefulness has expired. Once they continue on through the adventure I had a real fun twist at the end of book two... Once they kill the scarecrow I described it something like this... As you ascend the stair you hear a scream followed by a crash as a form plummets to the ground in the center of the clock tower kick up a large cloud of dust. As it settles you see a creature lying in a small crater with a serpentine tail and a woman's body, her face frozen in a scream. Looming over this form is a blood red creature. As it straightens you see it is female (feminine features) with four grotesque arms.. It's exposed mid drift shows a crisscross mass of scars, a large knotted scar across its chest (the fighter slashed nualia there), long silver hair, and burning yellow eyes. The creature turns and breaks out in a wide grin and in a raspy voice says "I knew you would come".... Then roll initiative. I made her stats just a little better than Xanesha. It was a fun and exciting fight I also included a picture I found on google images that is perfect... If you want I can put it in an email... I just can't figure out how to paste from my phone. Also a slightly modified Xill mini worked awesome. I think it was an epic fight, and they appreciated the nice tie in.