The affair of the mysterious letter

Upon returning to the city of Khelathra-Ven after five years fighting a war in another universe, Captain Wyndham finds himself looking for somewhere to live, and expediency forces him to take lodgings at 221b Martyrs Walk. His new housemate, Ms. Haas is a consulting sorceress of mercurial temperament and dark reputation. When she is enlisted to solve a case of blackmail, Wyndham finds himself drawn into a mystery that leads him to the drowned back-alleys of Ven and even to a prison cell in lost Carcosa.