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.
Beings of death are ancient and powerful, and it is dangerous to meddle in their realm. When Odyn peered into the Shadowlands, he saw some of its inhabitants: souls in torment, the husks of the dead, ghostly wraiths with no face, and others with no form, all made of death itself. This was enough to frighten even him. Several races in the Shadowlands, such as the kyrian of Bastion, are souls of deceased mortals who have been transformed to serve a new purpose. If one of these former mortals is killed in the Shadowlands, they die permanently. Other creatures—such as dredgers and stewards—are endemic to the Shadowlands and are naturally born from the magic of death to serve the different realms and help facilitate the process of the afterlife. If one such creature of death is killed, their energy is recycled back into the Shadowlands, and eventually another member of the same race will manifest to take the place of the one that was killed.
The Shadowlands will feature five major zones - Bastion, Ardenweald, Revendreth, Maldraxxus, and the Maw. In the center will be the city of Oribos, which functions as the main player hub similar to Shattrath City in Outland in The Burning Crusade or Dalaran in Wrath of the Lich King and Legion. There will be four new dungeons for leveling, four more at maximum level, and a new raid. In addition, a new roguelike "endless dungeon" called Torghast, Tower of the Damned, will be introduced for both solo and group play.
Covenants grant two additional gameplay abilities, one being class-specific. It alters your playstyle. Covenants grant unique armor styles and special capes that look a little more like accessories. Titan-like Kyrian capes look more like angel wings, for example. Each Covenant has a unique campaign similar to those found in Battle for Azeroth and Legion.
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.