I reconstructed it, and I have it equipped, I've been all around the maze and nothing in particular has happened. Does the mask work like infravision (eg I have to have one person selected for it to function) and where does it do it's thing?

It allows you to see an invisible guardian (looks like a fire elemental). That's all it's used for, but mages and thieves can use it to get the "Guard against Critical Hits" effect that helms give.

-Saz

Hm, interesting - I killed him before I got all the pieces of mask

A normal detect invisibility works as well. But the protect against critical hits is a nice ability for your mage or thief.

Actually, now I think about it, I probably had Keldorn's True Sight going at that time.