During the Facility and Nomad Valley areas you learn from the Oracle that a Kamari ship is heading towards the world you reside on, the Oracle can not communicate with the vessel and you have no idea of the intentions of those aboard.
As you reach the end of Nomad Valley to learn that the Oracle was wrong, the vessel did not head for the planet, it headed towards her outpost in the sky (the second sun that can be seen from the planet).
The Kamari aboard are rebels intent on destroying her, and in a last ditch effort she teleports you aboard her home base to hope you can put a stop to them.
The Second Sun features some pretty amazing new artwork, many corridors and several levels of gameplay.
You may even finally get to meet the real Oracle instead of her avatars.
Also inside the Second Sun is a vendor that gives out level 50 special Starforged Weapons.
Here is a sample of the Lower Level of the Second Sun:
The Level Cap has been raised from 61 to 65.