Quote from: Blizzard (Source)

Icecrown Citadel will feature a massive five-player dungeon sprawling across three wings of the citadel's foundation. While the Lich King's attention is focused on the Argent Crusade and the Knights of the Ebon Blade ripping through the front gates, players will be tested as they assist Jaina Proudmoore (Alliance) and Sylvanas Windrunner (Horde) in infiltrating the citadel through an alternate entrance.

An epic quest line will present adventurers with the task of weakening Icecrown Citadel's forces, requiring that players defeat the challenges in each dungeon wing before venturing into the next one. Normal and Heroic versions of the dungeon will be accessible to players, although each wing will be considered a separate instance; therefore, on Heroic difficulty, each wing will have its own separate lockout timer. All-new rewards -- including item level 219 (normal) and level 232 (Heroic) loot -- will be offered to those who destroy some of the Lich King's most formidable allies.

The Forge of Souls

Serving as the first wing in this expansive dungeon, the Forge of Souls will quickly put players to the test of carving through the Scourge stronghold into deeper, more treacherous locations. Jaina will command Alliance forces, and Sylvanas will direct Horde forces. The goal is to ruin the twisted engines known as soul grinders found in this portion of the citadel, and then players can advance -- that is, if the Horde and Alliance forces can overcome the foes who confront them.
Bosses

• Bronjahm, the Godfather of Souls: An instrument of reckoning, Bronjahm watches over the engines in the Forge of Souls. He must be killed if the soul grinders are to be destroyed.
• The Devourer of Souls: As the chief operator of the engines found in this wing, the Devourer stands guard over the souls stolen by the Lich King.

Pit of Saron
Accessible only to those who have laid waste to the Forge of Souls' unholy operations, the Pit of Saron will bring Horde and Alliance forces deeper into the Lich King's domain. Players who venture here will immediately be confronted by the lord of this lair, Scourgelord Tyrannus. But defeating him will not be as easy as it seems. Before they can present a threat to Tyrannus, the adventurers, instructed by their leaders, will need to free enslaved allies who have been trapped by the Scourge. Until that happens, Tyrannus will leave all adversaries to his minions, workers of the citadel's mines. Perhaps the challenges here will lend clues as to the whereabouts of the Lich King's private chambers outside of the Frozen Throne, deep within the Halls of Reflection.
Bosses

• Forgemaster Garfrost: A master of Scourge weaponry, the forgemaster hauls stocks of saronite ore and other precious materials to the cold forges where the mechanisms of death are born. With a host of rime weapons and exotic alloys at his disposal, it could get cold in here.
• Krick and Ick: Zombies serve as mindless muscle in the Pit of Saron's mines, stockpiling metals for Forgemaster Garfrost, and Krick -- a devious leper gnome -- supervises the operations from atop Ick, Krick's ghastly means of transportation.
• Scourgelord Tyrannus: Tyrannus is a terrible force who will no doubt demonstrate his powers to those brave enough to enter the Pit of Saron. The scourgelord must die if players hope to make their way into the third and final wing of this dungeon.

Halls of Reflection
With Jaina and Sylvanas leading the way, adventurers who make it as far as these frigid halls will quickly recognize the weapon that lies ahead: Frostmourne, the corruptive, legendary device of the Lich King himself. The Lich King's private chambers are within reach, although they may be the death of anyone who ventures there.
Bosses

• Falric and Marwyn: Captains for Arthas Menethil in life, Scourge commanders for the Lich King in death, Falric and Marwyn will be summoned to the Halls of Reflection for one purpose: destroying all intruders.
• The Lich King: Sylvanas, thirsty for vengeance against the corrupted prince who sentenced her to an existence as an undead monstrosity, and Jaina, eager to find a flicker of Arthas's soul locked somewhere within the Lich King, have brought their hand-picked allies to this final confrontation. Arthas's true power may only now be discovered. Is there any hope in this mission, or does only death await?