Allomancers have the ability to "burn" (or use) metals, in order to fuel a variety of physical and mental enhancements and abilities. For an additional burst of power, they may flare, or burn their metal especially quickly, consuming it at an accelerated rate, but gaining greater benefits in return.History
It is speculated that during the time of Alendi, Allomancy was in its infancy. There were no known Mistborns, and Alendi was one of the few Mistings in existence. When Rashek took the power of the Well of Ascension however, he condensed some of its energy into the metal Larasium which when ingested allowed him and several of his closest friends and allies to become Mistborn. The subsequent generations of these people were also Mistborn, and Allomancy slowly spread.
However, in time, Allomancers became less potent. People were no longer born capable of Allomancy, but had to be "snapped" into realizing their power. Usually this happened via an extremely traumatic event. Because of this, nobles developed a tradition of beating their children nearly to death in order to check for this ability. Also, Mistings who only had access to one Allomantic ability started being born. Even Mistborn were no longer as powerful as their ancestors.
Three hundred years after the rise of Harmony, Allomancy has diluted to the point where Mistborn no longer exist. However, Mistings are more common. A few Mistings are also capable of using one Feruchemical ability, these people are called Twinborn.