The Shadow on the Glass — Jonathan L. Howard

When two spiritist swindlers accidentally summon something horrific from beyond the stars, they must thwart a sinister time-spanning plot, in this first Victorian-era gaslight fantasy set in the world of Chaosium's Call of Cthulhu.

London, 1891. Elizabeth Whittle and William Grant enjoy scalping London's bourgeoise, taking on the persona of grand spiritist Cerulia Trent and her agent to connect the living and the dead. When a detective arrives, sniffing out fraud with a scientifically minded spiritualist society, the duo decides to take one last job before escaping to the continent. However, their final séance ends horrifically… and soon Lizzie isn't Lizzie anymore. William, desperate to banish whatever monster they summoned, is soon embroiled in an electrifying eldritch mystery where he makes a deal with the devil to save his friend and stop an even greater evil from transforming the known world.