Only the dwarves know how to “corrupt” a blade, strengthening it against things not of this world, but the dwarves did not make this dagger. We learn more about this in Indestructible, when Elothryn takes it to the dwarven priest to destroy it, without success.
The dagger is a tool, and we cannot talk about the tool without learning about the one who crafted it.
She goes by many names. The Tainted One. Taethza. Raezana. The beautiful one, chief of all the Ruwachs, an eternal being who seduced the mortal, Stygian. And in her rebellion, she kidnaped one of the twelve rulers of Zohar and fled the wrath of Vitkuko, the Shadow God. For her treason, she was thrown into the Abyss, but the seed of her seduction remained, and through it, brought about one of the greatest tyrannies known to the world of Nälu. The rule of Stygian.
How, you might ask, did she create such havoc from the depths of the Abyss?
She crafted a dagger so powerful that it cut through the barrier separating the world of the living from the underworld, creating a portal through which she could revisit her mortal lover, but only twice a year when the barrier was at its thinnest. But not without a price.
Twice a year, the dagger of Raemoja claimed innocent blood and opened the portal for the Tainted One to step through and rejoin Stygian in their unholy union. Stygian grew stronger, lived longer than any other being. And in each prolonged year, he grew more powerful, more severe, and cruel. He built the temple city of Raemoja to worship her, to provide the sacrifices, and to conquer the rest of the nations. To finish what they had started.
No one could defeat them. No one, that is, until the birth of the Chosen One who broke the covenant between the Tainted One and Stygian, releasing Nälu from their tyranny. But that is a different story for another time and place. The dagger remained. Should it be destroyed? Buried and forgotten? No. At the defeat of Nälu’s enemies, a prophecy was spoken. Translated from Táchil:
The Morning Star shined in the dark, blinding the sighted. Death and her loyal subjects defeated, but in shadows, they remain. Twice more Zohar’s Star will shine, but let not Death find him, nor the dagger. So listen well, Guardians of Nälu, guard the dagger that in slewing released the Star, lest they be found and the enemy unbound.
And thus the first Secret Keeper took the dagger and fled.