After the fall of last Old God Infinity Dragonflight trying to change timeline using Eye of Eternity. Bolvar joins them. Surviving dragon aspects ask Alliance and Horde for help. At Nexus Bolvar fights and corrupts Nozdormu creating Murozond. Battle ends in disaster. To stop them Wrathion with help of the remaining dragon aspects "unleash" Shadowlands on Azeroth (Northrend only) blending both together killing everyone in process.
New class based on death and time themes. Mail, 3 specialization - one melee (healer), two ranged (dps). Story - being DK who survived the "blending" and got affected by it (story wise only DK class characters survived cinematic). Class leader is Wrathion.
New zones are drastically changed Northrend after "blending" with Shadowlands. Plus island north from Northrend. One of the way to access new zones gameplay wise characters need to die.
Level cap squish to 50. 60 for expansion.
Cinematic starts with Bolvar reforging Arthas' old mace into what you seen on leaked shot. Then big battle with Nozdormu taking on Bolvar. Ends with fireworks. Narrated by Nozdormu/Murozond. Pre patch takes place in Northrend. Cinematic is result of campaign.
Some dead returning characters than we meet in new zones. Arthas plays big role.
Sylvanas don't make apperance or play any role in expansion for now.