The DLC launches on PC/Mac on August 31st, on Xbox One on September 15th, and on PlayStation 4 on September 16. Here's the official synopsis:
Located at the heart of the Cyrodiilic Empire, the Imperial City has fallen into ruin, claimed by Molag Bal's forces. It is comprised of six Imperial Districts, the extensive Imperial Sewers network, and the White-Gold Tower at its center. The dangers of Imperial City and the Sewers below are best suited for small groups of adventurers, but highly skilled warriors have been known to strike out on their own. While exploring and fighting your way through the Imperial City, be prepared to face not only Molag Bal's forces, but also enemy players from the opposing alliances.
Rewards that are unmatched in power await those who are brave enough to enter the Imperial City and claim them. Fight Molag Bal's personal guard, the elite Xivkyn, and other invaders from Coldharbour to win Tel Var Stones, the currency used to trade for mighty Veteran Rank 16 armor sets. But be on your guard—enemy Alliance members lurk around every corner throughout the Imperial Districts and Sewers. Should you fall to an enemy player, they can claim your hard-earned Tel Var Stones in their own quest for glory.