>>713557474
No that's in the base game. In chapter 6, I believe, Flayn is kidnapped and her blood is stolen. You can actually see Flayn and "Monica" unconscious behind the Death Knight when you go to rescue them.

While the students are busy with the Battle of Eagle and Lion, the Slitherers are busy. Solon uses Flayn's blood in Remire Village to begin testing for the Demonic Beasts. That testing is further pushed by Myson in Enbarr where Duke Gerth is surrounded by people infected with cursed blood and they're made rabid until they become demonic beasts too. Duke Och is blackmailed into stealing the Vajra Mushi from Kupala in Leicester, because the Slitherers are working on their zombie Ten Elites army and the people of Kupala learned how to make fake Heroes Relics. Which explains why the Ten Elites have their own and how they made the Aymr for Edelgard.

So yeah the Slitherers were really fucking busy. But before anyone could do anything about it, Edelgard declared war