During the Burning Legion's third invasion, the Knights of the Ebon Blade traveled to the Shadowlands to acquire the Aggregates of Anguish, five tormented essences of powerful souls - Admiral Proudmoore, Soulbinder Nyami, Grand Empress Shek'zeer, Grand Apothecary Putress and Archmage Arugal. Salanaar required these to conjure the Steeds of the Damned for the order's new Horsemen.
The experience curve is adjusted to make it faster than ever to prepare for the newest challenges, and each level along the way will provide more meaningful increases in progress and power. All expansion zones are getting more flexible, too; they'll be able* to scale to you and your friends while you level to 50, so it's easy to try a zone you've never played.
Furthermore, upon the Lich King's death he even spoke of "Seeing only darkness before him" while Sylvanas Windrunner said the same in Silverpine Forest after being risen from the dead by the Val'kyr. This may hint that because undead, death knights, and necromancers are connected to the Realm of Shadows when they die (again) they actually become apart of the Realm of Shadows and are forced to wanders in its dark mists for the rest of eternity. If this is so, then it can also be implied that when a death knight is given his/her own personal runeblade, the runeblade is actually used to bind the individual to the Realm of Shadows in mind and body, making it impossible to ever be rid of the death knight curse. In Howling Fjord, players are even able to witness the Lich King himself standing within the Realm of Shadows with two Val'kyr. The idea that necromantic magic and death knight runes drawing power from the Realm of Shadows itself is not yet proven, but it is heavily supported by in-game quests and lore.