Ethereal Fauna Silk
Silk taken from the Ethereal City hidden within the Mirror Cleft from whence the giants came. It carries the lifeforce of giants.
For giants, whose alien minds held no emotion, ambition, individuality, or form, this silk was the closest to a soul they ever came. And yet this trifling little thing was all it took for the Deep Fog to pursue the giants through endless dimensions and teach them fear.
It can be used to infect the Realms with the essence of the fallen tyrants. But only the mad or the wicked would ever do such a thing.