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!