Cold Iron

A rare variant of iron, only mined in the purest veins and carefully shaped. As tempering the metal will cause it to lose its strength, blacksmiths go through a lengthy process to shape it without causing irrevocable damage.
Cold Iron is used to harm Fae and Demons, which means that normally it is only needed in rare occasions. However, research by the Ravenmight Institute has revealed the likely possibility of significant Fae presence in the valley. Some of these are of a predator nature, which automatically puts us at odds with them. Others may be fair as long as one doesn't intrude, but the inherent corruption in the valley may very well have turned them too into a threat.
The Eastern Forest is most likely to contain Fae, to be more precise Dryads. While the Forester Woods are clear, the Dense Woods contain hints at Dryad presence. Since multiple magic users have reported a feeling from corruption there, we had best beware. As such, we are in need of cold iron, not to kill them but so we can protect ourselves and keep them at bay.
As for demons, neither in the present nor in the past can we find any indication of their presence. As such, it seems only Fae should be our concern. Other locations that may include Fae are the Northern Swamp and the Basin. Both places cause a feeling of dread, so it is quite likely that they too house corrupted Fae.
According to the scholars, the previous inhabitants of the valley did find several veins of cold iron. While we have only found a few scraps, this gives us hope that perhaps new veins can be found deep within the Depths. Before we run into a situation beyond our control, we should prioritize well-armed explorations to find such veins.