You can watch the fantastically-animated reveal cinematic above. It depicts World of Warcraft's own "Banshee Queen" Sylvanas Windrunner facing off against the current Lich King, Bolvar Fordragon.
For those who aren't versed in WoW's lore, Bolvar became the Lich King after his predecessor, Arthas Menethil (of Warcraft 3 fame), was killed by WoW's adventurers in 2008's Wrath of the Lich King expansion. The Lich King title is not a noble one -- his role (at least during Bolvar's "reign") is to prevent the legions of undead living in Northrend (the Lich King's domain) from breaking free of his control and assaulting the lands of the living.
Unfortunately, that task requires one to wear a helmet containing the spirit of evil Orc shaman Ner'zhul, which slowly corrupts its wielder over time. By breaking Bolvar's helmet, Sylvanas freed him from Ner'zhul's control, while also potentially breaking the tentative grasp the Lich King held over Northrend's undead.
So, where does that leave WoW's players? According to the developers, players will be venturing into the "Shadowlands," which is essentially Azeroth's afterlife. In the Shadowlands, players will journey through four separate zones, each more fantastical then the last -- there's the holy-looking Bastion, the gothic, vampire-filled Revendereth, the forests of Ardenweald, and the demon-infested Maldraxxus.
Specific gameplay details are still quite scarce, and we don't even know exactly why we're going to the Shadowlands (that information is usually revealed sometime after Blizzcon), but game director Ion Hazzikostas promises that player choice and agency is at the heart of this expansion's content. For example, you'll be able to align your characters with one of the Shadowlands' four "Covenants," which seem to correspond to the zones mentioned before.
Each Covenant has its own ambitions, motives, and powers that they will be willing to share with players. The other main feature we know of is the "Eternal Prison," a massive, highly-replayable, and ever-changing tower dungeon for groups of players to clear repeatedly.
One of the more interesting changes coming with Shadowlands is a different leveling system, and a level cap reset of sorts. In Battle for Azeroth, players were able to journey all the way up to level 120, but in Shadowlands, the starting level will be 50, with a new cap of 60. Blizzard says it wants to ensure that every level brings a "meaningful" sense of achievement, though the company did not elaborate further.
On the aesthetic front, Shadowlands will bring a "host" of new appearance options for all existing races, but we have no idea what they might look like.
If Shadowlands sounds like your cup of tea, you can pre-order one of its three editions right now. The $40 base package includes Shadowlands itself, as well as early access to the Death Knight class for WoW's Allied Races and the Pandaren. The Heroic Edition comes in at $60, and it nets you a level 120 character boost, and the "Ensorcelled Everywyrm" flying mount (which in turn lets you start a quest chain to earn the Vestments of the Eternal Traveler armor set).
Finally, there's the Epic Edition, which has everything mentioned before, in addition to 30 days of game time, the Anima Wyrmling pet, a new Hearthstone animation, and a cosmetic weapon enchant effect.
We don't know when World of Warcraft: Shadowlands will launch, but Blizzard says it's coming sometime in 2020.