Nicrosil
Nicrosil is the external enhancement pushing metal. An Allomancer burning nicrosil is able to enhance the effects of another Allomancer's powers. Feruchemists can use nicrosil to store Investiture, and when used as a Hemalurgic spike, nicrosil steals an unknown quality.
Allomantic Use
A nicrosil Misting is known as a Nicroburst. An Allomancer burning nicrosil and touching another Allomancer will cause the latter's reserves of the metals they are currently burning to disappear and give a huge burst of power.[1] This can cause issues with metals such as tin, where the mental overload can cause unconsciousness.
Feruchemical Use
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A nicrosil Ferring is known as a Soulbearer. Nicrosil is used to store Investiture. This ability is not understood (even by those who utilize it).[1]
Although the feruchemical uses and effects of nicrosil have not been canonically explained outside of the appendices in the books, the rulebook for the mistborn adventure game states that nicrosil can be used to convert stored feruchemical attributes into a generic form of investiture that can then be drawn upon to augment any feruchemical attribute. For example, if a feruchemist has stored up a lot of warmth but suddenly finds himself in need of some extra health, he can convert some of his excess warmth into generic investiture (which is shifted into his nicrosilmind), and can then draw upon the investiture as if it were health stored in a goldmind. The limitation being that he needs to have access to all three feruchemical abilities and their respective metals, this means that a simple nicrosil ferring is incapable of performing such a feat. However, the rulebook also states that any power gained from an external source, such as an allomantic duralumin or nicrosil boost, or even power gained from the well of ascension or the mists could be stored in a nicrosilmind for later use. This means that a feruchemists who travelled to other worlds could gain access to extra investiture. For example, feruchemist on Roshar could potentially store the investiture provided by highstorms in the same way that Returned can consume it in lieu of breath. Despite what can be inferred due to previously established rules on investiture, (such as all investiture being considered fundamentally identical) due to a lack of evidence, it can only be speculated that investiture stored in a nicrosilmind could be used to fuel other magic systems. For example, a twinborn using his feruchemically-stored investiture to power his allomantic ability, instead of having to burn a metal. Or even the reverse: a twinborn burning his metal, but storing the power in his nicrosilmind for later use, instead of gaining the immediate effect.
Hemalurgic Use
When used as a Hemalurgic spike, nicrosil steals some unknown trait or quality.