Lost to the Ages - p. 4 | Mutual side missions TES V: Skyrim - Dawnguard Guide
Last update: 11 May 2016
In order to open it, you will need to shoot two mechanisms placed on the right and left side of the passage. Aim at the blue crystals.
Head through it to reach the forge, covered by steam. Block it by turning the two marked valves.
New primary objective: Defeat the Forgemaster and forge one item
After you do it and approach the mechanism, you will be attacked by Dwemer robots. In the end the Forgemaster himself will appear as well
I'd recommend fighting by the wall of the room, as there you won't be hit by the steam appearing from time to time. You will be also able to easily shoot at the fire-breathing robots.
After clearing the area, approach the forge and use it.
Using it, you can forge one of three items: a shield, crown or staff. The required materials can be found in the chest on the right. I'd recommend thinking this over well, as you won't be able to change your mind afterwards. This way you will end the mission.