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.
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.
Certain playable races will receive new customization options (for example, humans will be able to customize their ethnicities, dwarves and trolls will gain tattoos, and undead will be able to show varying degrees of decay). The Death Knight class (added in Wrath of the Lich King) is being opened up to pandaren (added in Mists of Pandaria) and to all allied races (added in Legion and Battle for Azeroth); players who pre-ordered Shadowlands received access to death knights for these races with the release of Patch 8.3.0, Battle for Azeroth's last major content patch, on January 14, 2020.
The popular autobattler mode (games like Teamfight Tactics, Dota Underlords, and Blizzard’s own Hearthstone Battlegrounds) lets players draft units and place them on a chess-like board. The units then battle the units of another player. Between battles, players can buy new units, upgrade existing units, or rearrange them on the board. And according to Shadowlands’ most recent datamine on MMO Champion, World of Warcraft is getting a similar mode.
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.