Frostglint

Frostglint is a strange metal only found in rare areas where it gets extremely cold deep underground. It's typically hunted for by quarrying underneath glaciers, but most of the time the chill doesn't reach deeply enough for frostglint to appear. Typically, it's only found in areas near some kind of connection to the plane of water. The metal absorbs and seemingly destroys heat, making it both dangerous and highly useful.
Frostglint is often used in part of the construction of weapons by the smiths in Undyr, which adds a chilling bite to their edges. These weapons are famous in much of the surrounding territories, and is one of the many reasons Undyr's army is so widely feared.
It has also been used with certain other materials and minor enchantments to create a box that keeps a particularly low temperature for preserving food and other sensitive materials.
Undyr doesn't have full control of all sources of frostglint, but they do have a rather large portion of the supply. They exert their military and economic power on the other northern kingdoms to obtain their own in an attempt to keep it contained, and thus prevent one of their major advantages from making its way southward to their enemies.

A Legendary Blade

There are myths uncovered in ancient libraries that tell of a legendary sword by the name of Sun's Bane that was forged entirely of hardened frostglint. The combination of the sword's construction and the powerful magic contained within caused it to absorb and destroy heat so much so that it was a danger to even its wielder, and an utter terror to fire elementals, red dragons, and other creatures of flame. The reports of the blade's power are certainly striking, but even more so is how impossible its very existence should be. Because of the nature of frostglint, heating it enough to melt into a forgeable form has proved impossible with any smelting technique discovered in the Riftgrounds, making the idea of a blade forged entirely of the substance certainly an enigma. Where the blade may be, or if it truly exists, is unknown.