Rashek was the Lord Ruler, and a former Terris packman. He killed Alendi, the man believed at the time to be the Hero of Ages, and took the power at the Well of Ascension. He ruled the Final Empire, achieving immortality by combining Feruchemy and Allomancy. He created the Steel Ministry, its Inquisitors, the Kandra and the Koloss through use of Hemalurgy.
The power of the Well of Ascension gave Rashek the ability to recreate the world, but only for a limited time. He used this power to move the planet closer to its Sun, so as to burn away the mists that were threatening all life on it. This had the side effect of making the planet too hot to sustain life, forcing him to create the Ashmounts to thicken the atmosphere and protect the surface. This in turn forced him to change the people to be able to breathe the newly ash-filled air and to re-engineer the planet's biology to work with the new atmosphere. He also discovered Hemalurgy at this time and used it to create the Kandra, Koloss and Inquisitors. Finally he rearranged the planet's geography to hide the Well.
He condensed some of the power from the well into the metal Lerasium, which he then ingested, becoming a Mistborn. He is the only man known to have been a full Feruchemist and a Mistborn at the same time, and doing so granted him great power. One of these powers was immortality. As Sazed speculated in the Epilogue of book one, the Lord Ruler most likely spent time regularly storing "youth" in atiumminds (Vin saw him doing this when she entered his secret room in Kredik Shaw, just before she was captured). This caused him to age significantly during the storing process. However, because he was also an Allomancer, he was able to swallow and burn his atiumminds. This burning presumably released a great amount of youth, many times the amount that was originally stored. As Sazed explained in the Epilogue of book one, this Allomantic burning of the metalminds would not work for sustained youthfulness, only for a short, magnified burst. Therefore, when the Lord Ruler burned the atiumminds, he stored most of the resulting youth in his metalmind bracers (the ones that Vin ripped away from him in the final fight of book one). This allowed him to slowly draw on the youth stored in those metalminds, thereby allowing himself to maintain whatever state of youthfulness he desired.
The Lord Ruler was able to achieve amazing effects with his powers. Kelsier describes an instance in which Rashek was burned down to little more than a skeleton but regenerated in a matter of minutes and Rashek himself claimed he has been beheaded at least once. Although this potent combination gave him incredible power, allowing him to live for a thousand years, he became utterly dependent on his metalminds, and when Vin managed to Steelpush them away from him, he aged rapidly and died within a few minutes.
Some three hundred years after his death he is remembered as the Sliver, likely refering to his claims that he was a sliver of the power of god.
Rashek was born in a Terris town. His uncle was Kwaan. It was said that he hated all of Khlennium with a passion, but the culture of the Final Empire seems to have been heavily inspired by that of Khlennium, suggesting some degree of envy on Rashek's part.