Chromium is the external enhancement pulling metal.
Allomantic Use
A chromium Misting is known as a Leecher. Chromium acts just like aluminum inside the body, causing any Allomancers touched by a Leecher to lose their metal reserves.[1] Chromium is already widely known as of the events of Shadows of Self, as shown when Reddi mentions that there is a Leecher in the Elendel constables' employ.
A Leecher could prevent a Shardbearer from summoning their blade.[2]
Feruchemical Use
A chromium Ferring is known as a Spinner. Chromium is used to store fortune. A Feruchemist filling a chromiummind will be unlucky, and can tap it to increase his luck.[1]
Hemalurgic Use
It is unknown what trait or quality chromium steals when used as a hemalurgic spike, as chromium is discovered after era 1 of Mistborn. It is only in Misborn era 1 that hemalurgy is in widespread use, and therefore only in era 1 that a list of the hemalurgic uses for metals is listed in the Ars Arcarnum.
Speculation
In Mistborn Secret History, in a conversation with Alonoe in Ire (part 5, ch 2), it's mentioned that drawing on Fortune can cause coincidences to happen.