If you're looking for a book or series that combines a dark feel with nature-based magic, I highly recommend "The Black Witch" series by Laurie Forest. The series follows the story of Elloren, a young girl from a family of powerful mages who is struggling to find her place in a world where prejudice against those who possess certain magical abilities is rampant.
Throughout the series, Elloren learns to harness the power of her nature-based magic and must confront the dark forces that threaten her world, including dangerous and deadly creatures that lurk in the forests. The series also deals with deeply complex themes like discrimination, oppression, and societal unrest, adding an extra layer of depth to its already fascinating world-building.
Overall, "The Black Witch" series has a captivating and immersive storyline that is sure to pull you in from the very first page. Its combination of expertly crafted magic systems, nuanced characters, and mature themes make it an unforgettable reading experience that will leave you wanting more.