6dc5455c____The_Coppermind___17th_Shard__Entry
[Chapter] 58 [Entry] 
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.
The Hero of Ages/Epigraphs - The Coppermind - 17th Shard
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