Reserved for the cat - Mercedes Lackey

In 1910, in an alternate London, a penniless young dancer is visited by a cat who communicates with her mind to mind. Though she is certain she must be going mad, she is desperate enough to follow the cat's advice and impersonates a famous Russian ballerina. The cat, it turns out, is actually an Elemental Earth Spirit, and leads her to minor stardom.

Meanwhile, the real Russian ballerina has fallen victim to an evil troll who takes over her body and kills her patrons, drinking their life essences in order to strengthen his powers. And soon, the troll focuses his dark attentions on the young dancer...