In Baldur's Gate, she is seen for the only time in Chapter 7 at the top of the Iron Throne building. She will hail you on sight and question why you are here. No matter what you say, Cythandria will attack and summon two strong ogres called Arghh and Ughh. She carries Sarevok's Diary and letters advising of the location of Slythe.
When badly wounded she will beg for mercy. You have the option to accept or ignore this. The exact wording of accepting mercy is for her to "Give us all of your possessions and then get out of my sight". This will place the diary and letter into the inventory of one of your characters. If you decline and kill her, these items will be found on her body.
If you offer her mercy, she will attempt to walk into the side room and disappear, while walking, she will remain a hostile target. As she has very low health it is very easy to "accidentally" kill her at this point. If you do so, the diary and letter will not be on her body as she will have already handed them to you.