Total War: Warhammer 2's Mortal Empires campaign is set to arrive on October 26th, bringing with it an absolutely massive sandbox to play around in. To put it simply, the Mortal Empires campaign will be a combination of the maps from Total War: Warhammer 1 and 2, along with all of the Legendary Lords and the factions they lead! So if you've bought any DLC for Total War: Warhammer 1, all of it will be available as a part of the Mortal Empires campaign as well. That said, Norsca will be slightly delayed due to some technical issues, so I'm afraid you won't be able to conquer the jungles of Lustria with a horde of mammoths just yet.
If you're wondering how all of the different races might stack up against each other, especially when it comes to a battle between the new and the old, wonder no more as the developers have now posted a rather lengthy battle between the Empire and Skaven. Have a look:
To learn more about the Mortal Empires combined campaign map, or just Total War: Warhammer 2 in general, you should head over to Steam. The developers have done an excellent job of outlining all of the major features and changes, so you should get a pretty damn good idea of what to expect. Oh, and its also worth mentioning that all of this will come as a part of a free update!
And finally, just in case you were wondering, here's the video the High Elf vs Dwarf screenshot was taken from: