Ruin is the overarching antagonist of the Mistborn series, and serves as the main antagonist in The Hero of Ages. He is a Shard of the deity Adonalsium, and is the embodiment of destruction.
Originally, Preservation and Ruin were equal forces. However, Preservation made a deal and convinced Ruin to help him create sentient life (humanity) and Ruin agreed, on the condition that he would be able to destroy them later. Unbeknownst to Ruin, Preservation wanted to kill Ruin, and not being able to himself as the embodiment of stability, he hoped a human (Vin) would kill Ruin later on.
Preservation sacrificed his consciousness to imprison Ruin. This separated Ruin's consciousness from his body, which was in the form of the metal Atium.
In order for Ruin to be freed, someone had to release the Power from the Well of Ascension. To cause this, Ruin altered the Terris texts, to trick Alendi into thinking that the Hero of Ages must be self-sacrificing and give up the power. However, Kwaan figured this out and told his nephew, Rashek, to kill Alendi and take the power.
Rashek took the Preservation's at the Well of Ascension, and used it to remake the world and create the Final Empite. Rashek dedicated himself to stopping Ruin. When Rashek controlled the empire, he hid Atium so Ruin wouldn't be able to find it, and prepared as best he could for Ruin's eventual return. However, he also used Hemalurgy to create the Inquisitors, Kandra, and Koloss, which could be controlled by Ruin, as Hemalurgy is the power of Ruin.
The End of the Empire
Vin was born and Preservation wanted to use her as a vessel. However, Ruin forced Vin's mother to kill her youngest daughter with a metal earring, and give the earring to Vin, This earring acted as a hemalurgic spike which both allowed Ruin to influence Vin and also gave her increased Seeking power, since the younger daughter was a Seeker.
Kelsier led the Skaa rebellion against Rashek. Ruin helped him discover the "Eleventh Metal", malatium, which was given to Vin after Kelsier died. Vin was able to use the metal to realize the Lord Ruler, Rashek, was a Terrisman, When Rashek tore off Vin's earring, the power of Preservation, the Mist, was able to flow through her and she killed Rashek.
The Well of Ascension
After the fall of the Empire, Ruin was able to manipulate many people, including Vin, Marsh, and Zane. He ultimately led Vin to the Well, which refilled with the power of Preservation. In order for Ruin to be freed, he tricked Vin, just like Alendi, into thinking she must be self-sacrificing and give up the power. In an effort to stop this, Preservation stabbed Elend, hoping Vin would take the power to heal him. However, Ruin was still able to trick Vin into giving up the power, which freed him.
The End of the World
At this point, Ruin was much more powerful. He caused mass destruction, particularly in the form of the Ashmounts, which spewed and blocked out the sun, causing mass famine. He took control of the inquisitors and the Koloss. He sought out the Atium, which was his body. He did this by encouraging Vin and ELend to think that they must follow Rashek's clues to find it, since Ruin cannot see metal.
Ruin also manipulated many people, such as Quellion and Spook. He took the appearance of Kelsier and told Quellion to kill many people, and did the same to Spook to get him to kill Quellion and destroy Urteau. However, Spook, figured out he was being manipulated and tore out the hemalurgic spike from himself and Quellion. Spook then sent a metal message to Vin, to tell her that Ruin can control anyone, even with a small metal spike, but Marsh, under the influence of Ruin, intercepted it.
Vin led Ruin to Fadrex city, as both of them believed that it contained the Atium. Vin was captured by Yomen, and Ruin sent Marsh to recover the Atium. Vin realized that Ruin was after the Atium and fought Marsh. When Marsh tore off her earring, Vin took the power of the Mist to fight him off. Vin then tricked Ruin into thinking tthe Atium was in Kredik Shaw. Ruin had up to this point been using the Koloss to attack Fadrex, but he sent them to Luthadel to kill the rest of humanity.
Vin battled the inquisitors as Kredik Shaw. She defeated some of them, but was beaten by Marsh. Under the influence of Ruin, Marsh started torturing her. Marsh realized that the message he read that Vin's earring was being used by Ruin to control her. Marsh was able to break free from Ruin's control for a moment to tear off the earring. Vin took the power of the Mist to defeat all of the inquisitors except Marsh, Vin took in so much of the Mist that she became Preservation itself.
As Preservation, Vin battled Rashek, trying to stop his destruction, Meanwhile, humanity prepared for its last stand against Ruin's Koloss army. Elend and Sazed discovered the atium in the homeland of the Kandra. Elend took it for himself and all of the atium Mistings. They used the atium to battle the Koloss army. However when Elend ran out of Atium, Marsh killed him. This caused Vin to decide to sacrifice herself and kill Ruin. Both Vin and Ati, Ruin's host, died. Sazed then was able to take both the power of Preservation and Ruin and fix the world.