As the description says this is done in Chapter 15.
You'll play a short flashback and have to do the following:
*** Spoiler - click to reveal ***When you're done talking to Cerenna, walk up to her until you get two options.
- I can't kill her...
- ...And yet, I must!
For the Trophy to unlock you have to choose "...And yet, I must!". The other choice doesn't make it pop.