It is not Morathi giving birth to The Newborn, which is either the child of Slaanesh or Slaanesh escaping its prison by giving birth to itself. Morathi uses an artifact to absorb the souls of the Phoenix Kings that are in Slaanesh (so Finubar the Cuck is dead). She absorbs almost all of them until she encounter the soul of her love, Aenarion. He is disgusted by Morathi and stops her from consuming his soul. Morathi emerges stronger with the power of Phoenix Kings, but she still has her snake monster form. In fact, her two forms were separated, so now in the game, you play with both forms of Morathi on the tabletop.
Slaanesh's Newborn was able to escape into the Mortal Realms by the breach that Morathi made.
The other new lore is that the Free City of Anvilgard has been taken over by Daughters of Khaine, renamed Har Kuron. All of the humans and duardin have been killed, except for the Stormcast, because killing them would alert Sigmar to Morathi's betrayal. The only other Order factions in the city are all of the units that came from the Dark Elves army. The story ends with the Stormcast leader of Anvilgard being broken out of prison by someone with red eyes and bat-like wings. So most likely Malerion and his elves are going to show up and put Morathi in her place.
