Board Thread:Skyrim/@comment-108.163.127.221-20160126172503/@comment-50.147.180.22-20160206041755

I think he may have been captured by the Dwemer back when they were still around (i.e. pre-Brass Tower/Numidium incident). They would have kept him locked underground, and possibly forced him to breathe fire onto a boiler to fuel their machines.