Curmudgeonly scholar Emily Wilde must brave the beautiful but haunted Black Forest and open a portal to a long-lost, eldritch realm dreaded by the Folk, in this fourth installment of the New York Times bestselling series. Driven by an insatiable scientific curiosity, Emily Wilde has made a name for herself as the world's foremost scholar of faeries. Along the way, she's rescued a faerie kingdom and helped her now-husband--fellow professor Wendell Bambleby, dashing and infuriating in equal measure--regain his throne. And while understanding her fellow mortals has never been her strong suit, she's also made a small number of friends. But two years later, one of those friends goes missing in the beautiful but faerie-haunted Black Forest in Germany, a region of picturesque villages and sinister Folk. And if she's to save him, Emily will have to reckon with the darker side of her own nature, and how ruthless she's prepared to be to solve the most tantalizing of scholarly mysteries: the history of the ancient and long-lost realm she's named Dark Faerie, the only place the Folk fear. But Emily is not alone--she has her beloved Wendell, and her niece Ariadne, who hopes to follow in her footsteps. Told from the perspective of all three characters, this installment of the Emily Wilde series opens a magical portal to the most wondrous and dread faerie realm of all.