Turns out leaving Tar Goroth unattended like that was a bad idea after all. Go disrupt the ritual.
When you get there you'll have to deal with Zog. Again.
This can be an annoyingly tough fight- Zog throws spears and teleports, and there are spearthrowers and archers mixed in with the crowd as well. And, of course, they're mostly revenants, so killing them piecemeal solves nothing. Plus, the geysers rising from the ice can stagger you if you touch them, which you're likely to, because there's lots of them. The best strategy is to kite all the hostiles over to one side of the ice, then sprint back the other way and use the totem itself as a shield from their attacks.
Purifying all four totems removes the grunts and geysers alike, leaving you free to finally shoot Zog in the face.
Even without reinforcements, however, Zog is still a formidable fighter- he can't be vaulted, or ground executed, and his teleport attacks can't be countered. Best bet is to stay close and carefully whittle him down with swordplay and stuns, on the few occasions when he'll let himself be stunned.
It's definitely worth summoning help if you have it to summon- this is the last fight in the mission. Regardless of the tools you use, you can't dominate Zog, you have to kill him. Do so.
This richly deserved killing ends the mission.
It also gives you the Dragon Song skill, allowing you to summon drakes.
