A Fistful of Warlocks

A Fistful of Warlocks is a short story in The Dresden Files series by Jim Butcher. It is set long before Storm Front. It can be found in the anthology Straight Outta Tombstone.

Plot
Not yet commander of the White Council's Wardens, Anastasia Luccio rides into Dodge City, Kansas in pursuit of the warlock Alexander Page. Karl is her mount, a näcken disguised as a white horse after losing a bet with her. Stopping at the Long Branch Dance Hall and Saloon, where Wyatt Earp requests Luccio to give up her guns; she's however allowed to keep her sword.

Her hunt for Page is cut short by two warlocks, one with a German accent and the other a British accent. Their attempt to lead Luccio out of the saloon are stopped by Earp, who arrests the German warlock, bringing him to jail. In doing so, he proves himself to be a Venator. While waiting for the inevitable attempt to extract the prisoner out of jail, Earp produces a necklace, indicating that the other is involved with the Thule Society. Later, the British-sounding warlock requests the liberation of the prisoner, before being refused; Luccio realizes that necromancers are involved.

Karl reports that four of them are casting a spell, and fighting starts a few instants later. Karl and Luccio take on the necromancers, Page being one of them, Earp fighting the zombies the warlock have created and control. Slowly, Luccio and Earp manage to win the battle, pushing Page to suicide and killing one of the necromancers. The Briton and the German warlocks, however, manage to flee, but not before Luccio and Earp learn of their identities: the British warlock is a White Council's wizard, Grevane, and from available information, the German is Kemmler.