I love the Cosmere, can you give me the spoilers on what happened there?
Sometime significantly before Elantris, Odium shattered the two shards on Sel, Dominion and Devotion. Then he somehow sealed most of the power of said shards in the Cognitive Realm, which is why Raoden describes it like it's yearning to be let out.
The Reod itself wasn't necessarily tied directly to Odium's actions, but word of Brandon says it wasn't a natural occurence either. My theory is it's some kind of aftershock of the shattering and sealing of Dominion and Devotion.
It's also worth noting that Brandon has implied that Odium couldn't have taken on two other shards alone and had help from another shard. Prevailing theory right now is Autonomy, IIRC. He's also shattered Ambition.
Most of this is directly from Brandon via questions at signings and cons.