Ah! *insert light-bulb moment here*
Is it, by any chance, Katrina's island in QfG5?
By the way, I'm not sure I understand the ending of QfG4 and its implications on QfG5. The end of QfG4 implies that Katrina is pulled into the dimension where the Dark One dwells, trapped there much like Erana previously, and will continue to exist there along with Avoozl for eternity.
QfG5, on the other hand, says she was killed and went to Hades, from which the hero can rescue her.
So I'm guessing that Katrina was killed by her exposure to Avoozl?
That would make sense, at least. But then, how is it that Erana survived for so long, but Katrina didn't?
As magic-users, they strike me as fairly equivalent in power.