Dymitr is on a personal mission, and it starts with facing the mythical leszy who’s guarding the fern flower as it blooms for the summer solstice. “When Among Crows,” a novella by Veronica Roth — bestselling author of the “Divergent” series — dives into the world of Slavic mythology. A Chicago native tapping into Polish roots, Roth places nightmarish and intriguing creatures alike in the modern-day city.