In previous expansions, entities like the Loa of Death Bwonsamdi, ex-Forsaken Queen Sylvanas Windrunner, and corrupted titan watcher Helya all have spoken of the Shadowlands, and for some, the desire to conquer it. Sylvanas' character arc throughout Battle for Azeroth ends with her seeking to control death itself, indicating that she will play a pivotal role in Shadowlands.
The four zones that comprise the Shadowlands are ruled by "Covenants", similar to the Class Orders introduced in Legion. Each Covenant will have its own campaign, similar to the War Campaign in Battle for Azeroth, with gear specific to the faction and abilities both universal and determined by class. The four covenants are the Kyrian of Bastion, the Night Fae of Ardenweald, the Venthyr of Revendreth, and the Necrolords of Maldraxxus. Players experiencing the Shadowlands content for the first time will need to reach level 60 before they can choose to pledge themselves to a Covenant, while alts are able to do so at the start of the Shadowlands content.
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.
The final destination of the spirits of the deceased remains a mystery to the priests and philosophers of Azeroth. However, as spells such as [Resurrection] can reunite a dead body with its spirit, and a majority of living creatures from the tauren to the troll shadow hunters claim they can communicate with and call upon the power of the spirits, a widely held belief is that the spirits of the dead remain on the Material Plane — in an immaterial state that can only be altered or contacted through the use of magic.
But we know that the mission table feature will return in some way, thanks to a Hazzikostas interview with Twitch streamer and professional World of Warcraft tank, Sloot. In the interview, Hazzikostas confirmed that mission tables are present in Shadowlands, and that Blizzard wants it to be something you can do on your phone, rather than a front and center feature. Hazzikostas also stated that the team is looking to evolve the gameplay and offer more for players.
Sylvanas Windrunner, fallen leader of the Horde, has pierced the veil between Azeroth and the realm of the dead, setting in motion a series of events that threatens to upset the cosmic balance between life and death. World of Warcraft players entering the fabled Shadowlands will find the realms of the dead in upheaval. Under the normal order, departed souls were delivered to a realm appropriate to the lives they led, but now, all souls are being funneled into the Maw, where the most wicked are damned to suffer for eternity. As they seek to right the cycle and uncover the extent of Sylvanas' designs, players will forge bonds with the Covenants who hold domain over different planes within the Shadowlands.
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.