Blankšum

was a precious rare metal. Swordsmen from all over the world heard legends of the mystical metal and coveted its existence. Weapons and armour forged from this metal were unbreakable. In addition to that, was nearly weightless.

Shadow Labyrinth
In the battle against Dadaneuph in the Shadow Labyrinth, swordsmen wielding weapons made of  tried to destroy the demon’s core.