At Crossroad Keep (not the later battle I've accidentally read about), the fire ring shows up and then he just sits there and lets us kick the crap out of him. He won't summon the King of Shadows and when we (hubby and I) try to have Ammon/Zjeave or however you spell her name read the scroll, he's an invalid target (not a Shadow Reaver). Will reloading earlier in the courtyard battle help? That's our only save since right before taking down the towers from the walls.
I made saves at the beginning of the walls fight and at the end of it, then a save just before Black Garius appears. I don't remember exactly but I think I used the save at the end of the walls fight. The save just before Garius was corrupted so I can't use it. But the one at the end of walls fight works.
Anyway, I don't think that it is a real bug. It just an intense period of fighting so you can get erratic result ( such as no king of shadows avatar ).
This is a follow-up on this question. I ran another game and I did get into some problem with "no avatar". I repeated over ten times and finally I got the precious avatar and I was able to continue with the game. By restarting with a save game just after the walls fight and saving by increments I found that I can use a save game very close to when Black Garius appears and it still works. But It may requires a dozen repetitions before it works. So be patient!