Ancient mansions have long memories... and some never forget the taste of fear.

When people start vanishing without a trace from Thornfield Hall—a historic Chicago mansion renovated into an upscale event venue—Detective Sergeant Sofia Vega faces her first major case since her unprecedented promotion. Officially instated as the newest member of Chicago PD's Arcane Investigations Unit while secretly maintaining her position with the magical organization La Luna, Sofia now walks the razor's edge between two worlds as she investigates the mansion's dark secrets.

The evidence is disturbing: victims disappearing from locked rooms, security footage showing people entering hallways but never emerging, and the mansion itself seeming to reshape around its guests. Behind these mysterious events lies a century-old ritual designed by the mansion's original owner, Nathaniel Thornfield—a contemporary of H.H. Holmes with equally sinister ambitions.

As Sofia navigates the mansion's shifting architecture and hidden chambers, she discovers Thornfield Hall isn't just a building—it's a conscious entity hungry for perception and power. With her new official police credentials opening doors that were previously closed to her, Sofia assembles an unlikely team: technomancer Miguel Rodriguez, her skeptical partner Detective Jack James, and supernatural allies from Chicago's magical underground. Together, they race to prevent the completion of a ritual that could tear open the barriers between dimensions.

But the mansion's caretaker, Camila Reyes, has her own plans for Thornfield Hall. Continuing her ancestor's work, she's selected specific victims whose unique perceptual abilities will anchor a permanent doorway between worlds. When Sofia infiltrates a masquerade ball to protect the next potential victims, she finds herself trapped in a collapsing reality where walls breathe, doorways lead to impossible spaces, and an ancient consciousness stirs to life around her.

With time running out and hundreds of lives at stake, Sofia must make an impossible choice: destroy the mansion and everything in it, or attempt a dangerous purification that could save the victims but might cost her everything. Her dual authority will be tested as never before, as she balances the demands of magical and mundane law enforcement in a crisis that threatens both worlds.

The Unwelcome Party is the third novel in the captivating Chicago Underground urban fantasy series, perfect for fans of Jim Butcher's Dresden Files, Seanan McGuire's October Daye series, and Ilona Andrews' Kate Daniels books. Enter a world where magic pulses beneath Chicago's streets, where family traditions carry ancient power, and where some doors should remain forever sealed.