After reviewing Vaanic One's post in this topic, talking about how Dagoth Ur mutated, I realized that I have always been wondering, where did his mask come from? And why does he wear it?
I have looked in the construction set, and have looked at him in it and I know that he cannot take it off (at least in the game, I don't know about whether or not he really can). Just a little bit of curiosity.

It is purely ceremonial -- a symbol of power that he either forged himself, or otherwise received, and wears to exemplify his position.

As for what it is, in objective terms, I'd say it's a pumpkin -- and he can't take it off because he has really bad hat-hair.

A pumpkin... I see. Anyways, thanks.