For centuries, iron was viewed as a deterrent to all things supernatural, faeries and elves included. There are a couple of theories as to where this concept came from, but probably the most convincing one has to do with the smell of blood. Blood has long been tied to the concept of lifeforce, so the idea was that if iron smells of the same stuff, it’s probably abhorrent to unnatural beings. Another theory is that at a certain time in history, iron represented the future – it was the cutting edge (hah) technology of the day, something that the old powers just weren’t able to deal with. For the elves of the IZS universe of course, cold iron proved to be their downfall in a rather more tangible way.

↓ Transcript
A medieval man-at-arms is kissing his wife goodbye beneath a horseshoe that hangs tips up over their doorway. His kid hugs his leg.
Caption: Simple folk always tried to ward us off with cold iron.

We see the man-at-arms from behind as he rides off to war, broadsword strapped across his back. His wife and child wave goodbye.
Caption: I’d say it was foolish,

The man-at-arms stands over an elf-king who slumps on his elaborately carved wooden throne, a sword protruding from his chest as blood spreads out. The king speaks weakly.
“but it seems they were right.”