Mistborn #03 The Hero of Ages

We would be no adversary for a force like Ruin .
    A voice from the past seemed to whisper to her from long ago. What's the first rule of Allomancy, Vin?
    Consequence. Action and reaction. If Ruin had power to destroy, then there was something that opposed him. It had to be. Ruin had an opposite, an opponent. Or, he once had.
    "What did you do to him?" Vin asked.
    Ruin hesitated, frowning as he turned toward her.
    "Your opposite," Vin said. "The one who once stopped you from destroying the world."
    Ruin was silent for a long moment. Then he smiled, and Vin saw something chilling in that smile. A knowledge that he was right. Vin was part of him. She understood him.
    "Preservation is dead," Ruin said.
    "You killed him?"
    Ruin shrugged. "Yes, but no. He gave of himself to craft a cage. Though his throes of agony have lasted several thousand years, now, finally, he is gone. And the bargain has come to its fruition."
    Preservation , Vin thought, a piece of a gigantic whole clicking into place. The opposite of Ruin. A force like that couldn't have destroyed his enemy, because he would represent the opposite of destruction. But imprisonment, that would be within his powers .
    Imprisonment that ended when I gave up the power at the Well .
    "And so you see the inevitability," Ruin said softly.
    "You couldn't create it yourself, could you?" Vin asked. "The world, life. You can't create, you can only destroy."
    "He couldn't create either," Ruin said. "He could only preserve. Preservation is not creation."
    "And so you worked together," Vin said.
    "Both with a promise," Ruin said. "My promise was to work with him to create you—life that thinks, life that loves."
    "And his promise?" Vin asked, fearing that she knew the answer.
    "That I could destroy you eventually," Ruin said softly. "And I have come to claim what was promised me. The only point in creating something is to watch it die. Like a story that must come to a climax, what I have done will not be fulfilled until the end has arrived."
    It can't be true , Vin thought. Preservation. If he really represents a power in the universe, then he couldn't really have been destroyed, could he?
    "I know what you are thinking," Ruin said. "You cannot enlist Preservation's power. He is dead. He couldn't kill me, you see. He could only imprison me."
    Yes. I figured that last part out already. You really can't read my mind, can you?
    Ruin continued. "It was a villainous act, I must say. Preservation tried to escape our bargain. Would you not call that an evil deed? It is as I said before—good and evil have little to do with ruin or preservation. An evil man will protect that which he desires as surely as a good man."
    But something is keeping Ruin from destroying the world now , she thought. For all his words about stories and endings, he is not a force that would wait for an "appropriate" moment. There is more to this, more that I'm not understanding .
    What is holding him back?
    "I've come to you," Ruin said, "because I want you, at least, to watch and see. To know. For it has come."
    Vin perked up. "What? The end?"
    Ruin nodded.
    "How long?" Vin asked.
    "Days," Ruin said. "But not weeks."
    Vin felt a chill, realizing something. He had come to her, finally revealing himself, because she was captured. He thought that there was no further chance for mankind. He assumed that he had won.
    Which means that there is a way to beat him , she thought with determination. And it involves me. But I can't do it here, or he wouldn't have come to gloat .
    And that meant she had to get free. Quickly.

Once you begin to understand these things, you can see how Ruin was trapped even though Preservation's mind was gone, expended to create the prison. Though Preservation's consciousness was mostly destroyed, his spirit and body were still in force. And, as an opposite force of Ruin, these could still prevent Ruin from destroying .
    Or, at least, keep him from destroying things too quickly. Once his mind was "freed" from its prison the destruction accelerated quickly .
    58
    "THROW YOUR WEIGHT HERE," Sazed said, pointing at a wooden lever. "The counterweights will fall, swinging down all four floodgates and stemming the flow into the cavern. I warn you, however—the explosion of water above will be rather spectacular. We should be able to fill the city's canals in a matter of hours, and I suspect that a portion of the northern city will be flooded."
    "To dangerous levels?" Spook asked.
    "I do not think