If I remember right, then you have to talk to Morati in the dungeon of the castle first. In order to get her free, the Queen orders you to take down the dark elf rebels and to do that, she tells you to go to Twilight, which opens the access through the garden of the castle.
Strange, she was standing right behind the door and I could talk to her which triggered the quest afterwards. Where does she stand in the cell? Maybe you can get to her with the phase power if she is not right behind the cell door. But I did it her quest pretty soon though, so it could be that the trigger for her quest is locked to certain story progression and disappears when you are too far in the story, but not sure about that. And I guess you have not yet triggered her quest since you would see it in the quest area?