durnium

Durnium is an extremely strong metal alloy which is slightly heavier than steel. Durnium is actually an alloy between krallinite and steel/brass, which creates an extremely bullet resistent metal. Durnium is remarkably strong and invaluable in the creation of armour, as plates of durnium are strong enough to resist bullet. However, when durnium is put at a certain thickness it starts to massively weaken and break down and at that point, steel armour becomes better, which is the reason that durnium is not used to armour large vehicles aside from airships, but is always used for body armour and airship armour.
Durnium is made with krallinite, which can only be found in deep areas of the Underworld. Krallinite has been extremely scarce for all of history until it's source was discovered deep in the underworld by the Shorians. After that, experiments with krallinite alloys led to the creation of durnium in the year 1002.
Once durnium was discovered by the Shorians they immediately started arming their soldiers with durnium plate armour in preparation for an invasion. They did all of this is secret, and the world would only know about durnium after the Shorians used it's strength to re-conquer all of Borea and Arctus and establish the New Shorian Empire in the year 952. The Shorians also found out that durnium makes perfect armour for airships, as light, thin strips of it provide ample protection against airplane machine guns as well as many smaller guns on enemy airships.