Good day to you,
after killing the three druidic spirits, you’re instructed to return to the great oak.
When you do so, the final boss of the stronghold spawns.
I was playing a beefed-up alt and was spamming skills before the boss spawned and essentially killed him before the quest-progress tracker realised he had even spawned.
I have a screenshot of me standing over the loot from the boss in said location, with the quest tracker still saying “Slay the Infernal Tormentor”; sadly I seem to be unable to upload it here.
Had to log out and redo the entire thing, kind of annoying.
Hope this helps, thanks, keep up the good work. 