Wasn't this question already answered?
It is a helm from a plugin, you can download plugins from the official morrowind site (
http://www.morrowind.com) to add stuff to the game.
This is the description of this particular plugin:
"There is a quest attached to the Adamantium Helm of Tohan, a rare and powerful artifact lost in the Sheogorad Region. Word of it has begun to be heard throughout Vvardenfell. Travel north to Dagon Fel and talk to anyone about “latest rumors” to find out more about recovering the legendary Adamantium Helm of Tohan."
Greetings,
MM