Spirit healers decide when it's time for a mortal soul to pass into the Shadowlands. When a mortal dies and their soul crosses the veil between life and death, it is shepherded by the kyrian to Oribos so that they can be judged by the impassive Arbiter. All of the soul's contents—deeds, misdeeds, thoughts, accomplishments, and failures—are instantly laid bare before the Arbiter, who then judges in a mere instant and sends the soul off to one of the infinite realms of the Shadowlands, each of which is ruled over by a powerful Covenant. The Covenants are ancient and powerful orders who have existed since the Shadowlands were shaped long ago, and who bear sacred duties to help maintain the afterlife's ecosystem. Not every soul is filtered into the main four Covenants. Each soul brings with it a vital force known as anima, the product of all of the soul's experiences and actions in life. Anima is the lifeblood of the Shadowlands, making trees grow and rivers flow and is the source that's drawn upon to conduct the magic of death. Great souls—both good and evil—have a lot of anima, while those who have lived humbler lives have less. Four of the realms—Ardenweald, Bastion, Maldraxxus, and Revendreth—are especially vital to the functioning of the Shadowlands. However, there could be infinite afterlives, some small and tailored to a single person while others are vast and full of either splendor or torment. Between each realm is a cloudy space which was once rich with streams of anima connecting the different domains, but the current anima drought has caused the realms to become cut off from one another, breeding fear and mistrust among the inhabitants.
The Shadowlands are accessible upon death, where the player-character can roam, and can only be seen by other spirits. The Spirit of Tony Two-Tusk describes the Shadowlands as all grey and dark, swirling clouds. While Uuna was in the Shadowlands, she described it as dark and she was unable to see anything, though she could hear people interacting with her in the physical world. From within the Shadowlands, she could see the light of the naaru A'dal who was in the physical world, but it was so bright that it scared her. The moonlight at Lake Falathim allowed her to see a friend in the physical world.
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.
But it gets even more interesting when you look at how it will work for alts (second or later characters made by existing players). New characters made by existing players will have the option of leveling up the old way in all the same old zones from the start. But they will also be able to go through the new-player starting experience if that's what players prefer.
Claim a covenant's power: players will be called upon to forge a bond with one of four covenants that rule Shadowlands' new zones, setting the tone for their journey through the expansion with a full covenant campaign. As they level up, they'll get new abilities based on their choice, including class-specific abilities for each Covenant—as well as developing soulbinds with specific covenant members, which grant them access to that character's specific traits and bonuses.