"In the beginning, those two were equal. Nothing of import happened or for that matter existed, because they were equal opposites and whatever one did the other would undo. Eventually they reached an agreement. Preservation wanted to create humans, and Ruin would refrain from immediately destroying them because sentient life contains more of Preservation than it does of Ruin. The fact that some of Preservation's power was placed in humans is why Ruin is now more powerful, and the fact that it's more powerful is why it accepted the deal. It was assured of being able to destroy the world eventually.

Preservation betrayed its counterpart. The prison it built keeps Ruin's mind separate from its power, so it can't affect the world except very subtly. Its power is still out there and people can use it; it's stored in a valuable metal best known for its usefulness in combat.

But even though their powers are usable, Ruin's mind is imprisoned and Preservation's is dead, so neither is in a position to accomplish their goals directly. The end result is that the prison needs to be re-created and Preservation can't do it itself. Why it's temporary in the first place I don't know. The side effects of the holder becoming more like Preservation are just a consequence of having something of that scale attached to your mind.

That's all from the mythology of an ancient religion that was followed by the Terrismen. During Ruin's last attempt to get out, it managed to help the real Alendi conquer the world, and arranged for him to fit the Terris prophecies about the Hero of Ages. It changed those prophecies to say that the Hero must lay down the power and resist the temptation to use it. All very noble, but had Alendi done so it would have freed Ruin. I stopped him.

And having been Preservation and seen this for myself, I can say that the mythology is correct at least as to how the imprisonment works."