Todays Rule of Three post on the WotC site gives us a little more detail about Rise of the Underdark storyline
Deep in the Abyssal darkness of the Demonweb Pits, the goddess Lolth is spinning a web of deceit, treachery, and ambition. Her goal is to seize control of arcane magic—a position that has lain vacant since the death of Mystra a hundred years ago.
To facilitate this grasp for power, Lolth sent a prophet—Danifae Yauntyrr—to all the cities of the drow. Danifae is a fallen priest, a scoundrel, a seductress, and—if history is any guide—an avatar of Lolth herself. Danifae whispered to the leaders of all the great drow houses, spurring them to gather ancient power to aid Lolth’s ascent.
The drow are scouring the world for primordial relics (pieces of a dead or sleeping primordial), seeking out the remains of great wizards, gathering artifacts once sacred to Mystra, and fighting for control of magical locations on the world’s surface.
As the drow gather arcane energy and channel it to Lolth, her power grows. Her web extends to cover most of the world, forming a Demon Weave to replace the sundered Weave that Mystra maintained—a new fabric of arcane magic. With that, the priests and wizards who serve Lolth have access to greater power than ever before, and their task becomes to use it.
Across the surface world, Lolth’s servitors draw on the Demon Weave to create shrouds of darkness that cover league upon league of surface lands. Under this pall of darkness, the drow can move and fight freely during what would normally be daylight hours. With this aid, they can complete Lolth’s transformation and bring the world to ruin.
It also clues us in to a few more events and products linked to the storyline: The GenCon championship adventure – Dawn of Night, and online support in Dragon 413 and Dungoen 204 (3 issues away).