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.
After being deposed as Warchief of the Horde in Battle for Azeroth, Sylvanas Windrunner travels to Icecrown Citadel in the Scourge-infested wastes of Northrend, and confronts the reigning Lich King, Bolvar Fordragon. Taking the Lich King's Helm of Domination from him, she tears it in two, shattering the veil between realities and opening a portal to the Shadowlands, Azeroth's afterlife. The Shadowlands have become warped in function during recent times. The Maw, a place in the afterlife reserved for the most sinful souls, is now absorbing all souls.
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.
In the opening cinematic, Sylvanas shatters the Helm of Dominion, breaching the veil between the material realm and the Shadowlands, an ancient dimension where the dead coalesces for judgment. Sylvanas' actions have upset the natural balance between life and death, and it's on players to enter the Shadowlands itself and repair the cycle before it is too late.
The Shadowlands has long been referenced in World of Warcraft lore, but their origins and true nature remain a mystery. One thing is for certain; the Shadowlands are inextricably tied to death and decay. It stands opposed to the verdant spiritual realm of the Emerald Dream, which represents a vision of reality untarnished by corruption and mortal intervention.