Aethersteel is a magically-infused metal originally created by the druids of Thyrelume. Its creation has been lost to the modern world.
Aethersteel is extremely light and strong. It could be used to create submarines, sand skimmers, or other forms of technology that no longer exist in the world.
However, smithing with aethersteel requires an extra level of protection. If inhaled or contacted, it can leave a person in a weakened state.
In Eveninglow 1732, the adventuring party discovered notes in Thyrelume with a recipe for using ettionga flower root to treat exposure. They used this to cure Edel Agasson.