Mistborn: The Lost Metal

The Lost Metal is the tentative title for the fourth and final book set in Mistborn Era 2.

Possible Information
The Lost Metal may refer to atium or lerasium, possibly even harmonium (ettmetal) or trellium.

When Kelsier destroyed the Pits of Hathsin, it was stated that the atium geodes would grow back in 300 years time, which may further contribute to atium being the lost metal. Additionally, while at a party Waxillium Ladrian sees each metal in a glass case with a plaque labeling them. Atium's case is empty with the plaque saying "Atium, the Lost Metal."

Ettmetal powers many southern Scadrian technologies and has much potential to be discovered in this area. But it only appeared after the Catacendre and is not known to the northern Scadrial until the events of the Bands of Mourning. This means it is probably not "lost" but "new".

Trellium is the metal that created Paalm's or Chimeras' single spike. Miles' last words also mentions the "bearer of final metal" and that he worships Trell. But since Harmony doesn't know it, it may not be a "lost" metal but a "new" metal.

Behind the Scenes
Following completion of The Alloy of Law Brandon plotted out three more books, following the same characters, to finish the story. The last of these is The Lost Metal.

It is estimated to be released in 2021, having been delayed from its original planned release due to the Skyward series.