Decay: Who Was He?

[Lilamal]

Who was Decay? He wasn´t quite like the Fell Twins, and he was definitely similar to their father, who we don't know exactly what he is, but is likely a Shard. So what was he? He could be a Shard, a worldhopper, or something else entirely. We don't even know if this is before or after Adonalsium's Shattering, so he could be any number of things we've never even heard of! Theorize away!

[Pagerunner]

Decay was an early version of Ruin, as officially confirmed here by Peter: http://www.17thshard.com/forum/topic/21542-the-status-of-aether-of-night/. When Brandon was building Mistborn: The Final Empire out pieces of his unpublished works, he took Decay and the war-between-Shards and turned it into Ruin and the conflict with Preservation.