Adventurers entering the Shadowlands will find the realm of the dead in disarray. In the natural order of things, souls are sorted and sent on to an afterlife realm appropriate to the lives they lived, but starting around the time of the Burning Legion's third invasion of Azeroth, all souls who have perished—including the innocents slain at Teldrassil—are being funneled directly into the Maw. The Shadowlands are starving for anima even as the Maw continues to grow from the glut of fresh souls. Sylvanas Windrunner has been seemingly perpetrating acts to bring about great amounts of death and destruction. In partnership with the Jailer, they have been working toward a common end for some time.
When the time comes to ally with one of these four, you’ll want to carefully consider the abilities the Covenant will provide you. Each provides two abilities tailored to the theme of the Covenant. The first of these is the Covenant’s signature ability, which is available to all Covenant members regardless of class, race, or specialization. This ability helps you explore and engage with the world in new ways, and gives you a unique tool to help solve the problems you encounter in the Shadowlands. The second of these abilities is specific to your class, and gives you a new spell or power to use in combat.
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.
As you climb the tower, you may discover a floor that contains a Legendary chest. Inside you’ll discover reagents to craft legendary items that can be taken to a mysterious rune sage who will help you build legendary items. You are in full control—you can choose which type of equipment you’ll make and what powers will go in them to make them truly your own.
The Shadowlands were originally intended to be a fully playable zone in World of Warcraft, designed for high-level play. The idea was that as deceased characters made their way back to their corpses, they'd see high-level players battling creeps in the area, which would inspire lower level players to increase their character level so they could experience the zone for themselves.
Shadowy Tormentors inhabit this dimension, and emerge from it to torment death knights such as Koltira Deathweaver. The Lich King originally kept them at bay, but after the Knights of the Ebon Blade left his service, the tormentors are now loose upon them. He even appears in the World of the Dead, as he calls it, riding a frost wyrm during  The Power to Destroy.