I like the idea of calling it "Mythral", but it doesn't matter too much what it's named. The idea is that there be something harder to get and more powerful than steel, but less tedious and deadly to get and less powerful than adamantine. The material could be discovered in the depths of the new rift near Krofburg, perhaps? Amidst some seriously troublesome creatures but not quite as deadly as the fiends of Perfidus, and be far more straightforward in its harvesting.
Weapons crafted from this material = same damage bonus as steel, +1 attack. (no further bonuses like adamantine's +2 vs construct), 60% weight (instead of 80%)
Armor and shields crafted from this material = 60% weight (like thick chitin), skill penalties reduced by 1 due to armor being thinner and more easily jointed than chitin (this can be done by granting +1 to parry, hide, ms, set trap, pp, et cetera, so that a chain shirt has an effective skill penalty of -1 instead of -2, tower shield -9 instead of -10 )