After initiating the final fight for the green fire quest line, Kanrethad goes through his phase where he taunts you, then goes to summon the pit lord. The pit lord, however never appears, and Kanrethad continues to his casting phase. I was hoping to do this quest at Level 91, but the pit lord never spawning makes it impossible until later levels. I have quit the game and reloaded and tried not damaging Kanrethad. None of which seem to work.
Edit, Solved by attacking Kanrethad while standing near the portal he comes out of.