Flowers of Perun backstory was in Every Leaf In Springtime.
Pip: So, ah, what’s she testing?
Seras: Perenova tsvetje: the Flowers of Perun.
Obtained by Sir Integra in the wilds of Poland, after a nightlong vigil and a battle with the forest itself!
Pip: She fought with trees?
Seras: Well, they were really dangerous trees!