Gate of pale reflections/ The heart of corruption

So, when using the champions essence on the roots in the heights of Athim the roots disappear but the quest marker remains. once passing the roots and heading into the boss arena the caretaker has a brief dialog but the final boss doesn't appear. I've tried everything from changing the difficulty, loading an earlier save and uninstalling the game but for some reason the final boss never appears no matter what I do. 

Any help with this will be much appreciated.