Ruin

Ruin is the destructive god who is released at the end of "The Well of Ascension" when Vin releases the power at the Well of Ascension. Eons ago, Ruin was tricked by his brother, Preservation, who created life, which had more of Preservation than Ruin. They had made a deal that Ruin would eventually get to destroy all life, but Preservation sacrificed his mind to bind Ruin's consciousness into a prison, separate from his main strength, his body. Ruin's body is the metal Atium. In this form, Ruin could only weakly affect the world. The Lord Ruler was aware of Ruin, and also that Atium was Ruin's body.

Spoiler warning! This article contains plot details about Books 2 & 3 and will spoil part of the ending.

When Vin releases the power of the Well of Acension, the prison around Ruin was broken and he was let out into the world. Though lacking his body, he could still influence anyone who has metal piercing their body. Hemalurgy is of Ruin, as it takes existing attributes and transfers it in a reduced form to another. Ruin could easily influence creatures with Hemalurgic spikes in them. Ruin needed to be 'concentrating' on a particular individual to control them, the amount of 'concentration' necessary decreasing with the amount of metal in their body. Creatures with a great deal of metal in their body, like Steel Inquisitors, were almost under complete control of Ruin, while those with fewer spikes were harder to control. Ruin could also directly control the Koloss, as their minds are very weak. To people who were pierced with single pieces of metal (like Vin and Spook), Ruin could appear as a voice or an image in their mind, and nudge their thoughts in the directions he wanted. At the end of the third book, Ruin's current host is killed by Vin. Ruin's host apparently had flame red hair. Sazed, the Hero of Ages, absorbs the powers of both Ruin and Preservation, and goes on to change the world.