It's deep inside the cave where you find the vampire journal.
You just need to open all 3 chests.
Not that is needed I restarted the game and played it 3 new games last few days to play all 3 evil mode character for the 1st game I had that problem I still have that saved in slot 1 quest was not completed and all three chest was opened so couldn't open the chest again thus unable to complete the quest at all.