Similar to Ulduar, Throne of Thunder is a raid that is going to feature a final boss that is heroic only. And much like Algalon, we know next to nothing about Ra-Den, the new heroic-only boss available after only defeating Lei Shen on heroic. Now, i figured there may aswell be a thread devoted entirely to discussion about what this boss could be. Lets start.
He is referred to as the Keeper of Storms, and that none have seen him in battle. Going by the title and how he is only available after Lei Shen, im going to say he is a being associated with thunder. His name only assures this.
Now, one thing noticeably absent from Pandaria is any titanesque beings. The mogu ARE NOT physically related to the titans, they just dtole their technology. This is very troubling, as seeing Un'Goro and Scholozar (Sp), who had that random titan thing and Freya, the Vale of Eternal Blossoms SHOULD have a watcher. The Titans wouldn't leave such and exploitable place unguarded.
Back to Lei Shen and the Mogu, we have almost no idea how Lei Shen/Mogy rose to power, other that Lei Shen found an artifact in the mountain, and used it to rise to power and unite the Mogu. Lo and behold, Blizzard is adding in more Lorewalker stories detailing the Rise and Fall of the Mogu.
Now, my theory. Blizzard is going to retcon what they said about Mogu, and we are going to find out that they were infact Titan creations( similar to Vykrul), who got affected by the Curse of Flesh. After this, they became very hostile and prideful, and seperated into clans, as they no longer could/wanted to do their job, watch over the Vale. They were controlled by a Titan watcher, who after seeing the Mogu's rebellion, begins to purge the Mogu, hence their aggression. This watcher was called Ra-Den by the Mogu. Lei Shen one day found an artifact left behind by the Titans in a vault that allowed him to siphon and restrain Ra-Den, and possibly other Titanic creations.
The Thunder King was only defeated because this artifact was either lost or stolen.