What's the East Rift? Well, I'm glad you asked!
FWIW, you guys are on it. It's what's left of the Great Rift after the Spellplague. From Countdown to the Realms:
In many places, the Spellplague wrought drastic changes to the very shape of the world. The vast Underdark system beneath the western Shaar suffered a calamitous collapse, leaving a miles-deep pit the size of a country where the Landrise once ran.
This event formed the East Rift. There's much more to it, I'm just not at liberty to discuss it (yet) but I really wish I could. I'm in charge of making sure cool bully adventures get written for the area in Living Forgotten Realms.
Once more becomes available to share, I'll certainly share it.
Bill Benham
LFR Pacific US (East Rift) Writing Director