Once you arrive at the city of Rosberg in Aedirn, this quest will start, with Rayla expressing her anger at the Nilfgaardians taking the city. Nilfgaardian scouts see Meve, so The Battle of Rosberg starts.
After the battle ends Meve will start to leave the city, though first she meets Xavier Lemmens, who she saves from the fire. He explains why the city fell, blaming it on Scoia'tael and then asks to join Meve's company, which she excepts. So then Meve and the rest of her army continue on to Aldersberg, to find King Demavend III.
Sadly, it was Rayla who met with surprise. A hundred-yard gap gaped in the "unbreachable" walls of Rosberg. The fortress itself was aflame, spewing black smoke into the sky.
"I don't... how?! It can't be!"
There was no time to consider her question, for Nilfgaardian scouts had spotted Meve's company. The queen knew battle was inevitable, so she gave the signal to form a defensive line, then drew her blade.