Mithral

Mithral is a very rare silvery, glistening metal that is lighter than iron but just as hard. When worked like steel, it becomes a wonderful material from which to create armor and is occasionally used for other items as well. Most mithral armors are one weight category lighter than normal for purposes of movement and other limitations. Heavy armor is treated as medium, and medium armor is treated as light, but light armor is still treated as light. Arcane Spell Failure chances for armor and shields made from mithral are decreased by 10%, maximum dexterity bonus is increased by 2, and armor check penalties are lessened by 3 (to a minimum of 0).

An item made from mithral weighs half as much as the same item made from other metals. In the case of weapons, this lighter weight does not change a weapon’s size category or the ease with which it can be wielded (whether it is light, one-handed, or two-handed).

Mithral items have the same durability as normal (steel) items.

Summary:

 * 1/2 standard weight (The DDO tooltips say the weight is 1/2. However, the actual weight is still normal)
 * -10% Arcane spell failure (to a minimum of 0)
 * 3 reduction to Armor check penalty for armor (to a minimum of 0)
 * +2 increase of the maximum dexterity bonus to AC