Elden Ring - Fia's Champion,Lichdragon Fortissax Boss Fight & Location Missable Achievement (Trophy) 

@CR -956 You need to progress her quest first for it.

Killing fia doesn't matter if you're still looking for an answer

go collect the Cursemark of Death which is located in the Carian Study Hall then go back to her and there should be an option to give the item you just collected to her. Then just talk to her until she repeats her dialogue reload the area and the go talk to her again. Then Reload the area. Worked for me

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