By breaking the Lich King's Helm (in the video above), Sylvanas also destroyed the only item that was keeping the countless hordes of Scourge undead in check. Now, with the release of the latest update, these same undead have invaded Azeroth, in a pre-Shadowlands event that is divided into two parts.
In practical terms, the event means that Orgrimmar and Stormwind were invaded by undead, but there is also a whole new questline to follow and rewards to collect. Some bosses from the Northrend era will also appear in the world, and it will all culminate in a confrontation against Nathanos Blightcaller.
Shadowlands will be released on November 23.