The Furnace

A giant jumps you on the path in The Furnace, and that sets the whole task for this place: you don't kill it, you shut it inside the furnace. Contact with the giant ends the game, so save more often here than anywhere before.

The main lever won't do anything on its own – it has a condition. You have to lead the giant into the furnace and close all of its gates behind it from the outside. Only then use the main lever in the room on the other side and wait. We cannot confirm where the controls sit or the order to use them in, so on your first pass walk the place and note what each gate and each control does before you start leading the giant anywhere. This is the trigger condition, not a written-out escape route.

With the furnace dealt with, go back to the path where the giant first attacked you and follow it to the Anvil of Creation. Your party then returns automatically to report to Durok.

Durok goes off for a while; when he's back he hands you Soulcleaver, Axe of the Deep Forge. Pick that weapon up before you enter Underkeep Tower. There is a claim that the tower's underground only opens after you beat the Archmage if you collected the Soulforge weapon before going in; we cannot confirm it, but collecting your reward early costs nothing.