"The Last Gambit" Mini Boss isn't spawning

Elgar'nans Wrath in The Last Gambit doesn't spawn. If I reload the save, the game says there's an error while loading and my Rook becomes bald. If I try to load another save, there's an error and I get back to the main menu. My usual workaround where I load saves from the lighthouse, doesn't work either. I have to play the quest from the start again but the mini boss just won't spawn.