Isiidium
by Jamie BatyIn one of the Immortal-level adventures, you go to the plane Rylum (I think), and it has a metal known as Isiidium that granted you a +3 bonus to hit and a +6 bonus to damage if your weapon was made from it. In the description it even mentioned that a few weapons made from it had made their way to the prime plane, and of those, some were magical as well.
Well here is some more info from my project:
Isiidium
Isiidium is a metal that far surpasses even adamantine in performance. Weapons from this metal are sharper than razor sharp and armour is nearly indestructible. Like red steel, normal weapons made of isiidium may hit magical creatures, even if it is not enchanted. It is extraplanar in origin- from the Plane of Rylum and a land known as Simoom (see IM2- The Wrath of Olympus). Not only is it not native to Mystara, it cannot even be forged on Mystara. Any Isiidium weapons and armour in Mystara came already forged from Rylum. Very few magic-users and other planar travellers know of Rylum, so only a few items come back to Mystara. Immortals sometimes will give them as rewards to devoted followers. Magical weapons and armour made of isiidium are incredibly rare- the Isiidi do not freely or easily let them go. Any magical isiidium item found has probably (95% chance) been enchanted after the fact on the Prime Plane. Because of this, they cannot be enchanted beyond +3, they cannot be intelligent. If the item was enchanted on the Plane of Rylum (5% chance), the item may be up to +5, be intelligent, and have talents, as a normal magic item would.
Isiidium: Cost Multiplier- x 500; Encumbrance is unaffected. Weapons are +3 to hit and +6 to damage; +3 bonus to armour class (i.e. Isiidium chain mail is AC2).