Atium
Atium is one of the two god metals. An Allomancer burning atium is able to see into the future by a few seconds. Feruchemists can use atium to store age, and when used as a Hemalurgic spike, atium steals any property or power.
Atium is notable as it is the condensed power of the Shard Ruin. It was sequestered away at the Pits of Hathsin by Preservation when the latter gave up his mind to trap Ruin.
Allomantic Use
A atium Misting is known as a Seer. Burning atium allows the user to see a few seconds into the future. If another Allomancer burning atium is encountered, the atium ghost will split, since each seeing into the other's future means that they can both make different decisions that affect both Allomancer's outcomes.
Feruchemical use
Atium, when used by a Feruchemist, stores age. When storing age, the Feruchemist spends time older. It is possible to then burn an atiummind to get an enormous burst of youth - allowing for practical immortality.
Hemalurgic Use
When used as a Hemalurgic spike, atium is like a wild card. Depending on where it is placed, it can steal any Hemalurgic property.