The Writer glimpsed it in his journeys through the various hells, but he paid no heed to it. His tale was about the afterlife and the punishments therein, not the arms and armor of the Darkness.
The grave robber grinned as he left the royal barrow, his pockets full of stolen gold, and dressed in a helmet and chainmail shirt stolen from the now naked, decomposing body of the king. The explosion that followed ten seconds after he stepped into the sunlight wiped the smile from his face and blew his body into pieces. Had he studied metallurgy, he would have known that the armour was made of pure Orthacarium and he would have left it alone, and escaped the barrow with his life.
With regard to the plot "Sleeping Mines of Elathon":
You manage to get the water wheel in the mine working which operates a pump to clear the flooded lower levels. As you explore the dank tunnels of the lower galleries, you can hear the creaking of the wheel above. You enter a low-ceilinged passage and are crawling on your backs, when the creaking stops. The water starts seeping wetly around the back of your head...