This is a Missable Achievement and can be obtained by a certain choice in the last Quest.
*** Spoiler - click to reveal ***
If you already completed the Main story,you should have a Savegame “Capture the Castle”, if you didn’t overwrote it.
While your on your last Mission [The Night of Longfangs] you have two Choices: Find Syanna or find the Unseen Elder, you need to pick the first Task (still the Unseen Elder is mutch more interesting Story and the Game has a better Ending).
When you found Syanna [Beyond Hills and Dale...] you have a few Tasks in the land of a thousand fable to complete. To get this Achievement you can’t pick up the Ribbon from the Girl who sells Flint, as the Questlog tells you to.
Later when you meet with Dettlaff, he then tries to kill Syanna (since she is not protected from Evil anymore, he succeed). Anna Henrietta will then trough you in Prison, for your failure to keep Syanna alive.
Thanks
@D33AN for the Hint, still took a few playthrough of this Quest to figure it out.
Posted by Rizzo NP: This didn't unlock for me the first time. I rebooted my Xbox, played through the last bit again and it unlocked when the prison scene started.