It brings an increased level cap, revamped instances and zones as well as new ones, news skills and "functional pets" for players with perks.
Here's an excerpt from the NCsoft press release:
"In Aion: Assault on Balaurea, the exciting new chapter of Aion, characters have the ability to align together to launch massive counter assaults against the invading Balaur on their homeland while experiencing new and updated instances and zones, an increased level cap, new weapons and items, new skills and added flight mechanics."
"Characters will not be alone as they explore the new areas, as Aion introduces functional pets equipped with in-game benefits." The boxed retail copies of Assault on Balaurea include "exclusive in-game pets" not available with the digital versions.
Check out the preview trailer for Aion: Assault on Balaurea.