In the RPG
Phlogiston is the miracle substance driving most technology in Azeroth. Phlogiston is a gas collected from the air and distilled into vials of liquid phlogiston; Produced by phlogiston collectors, vials of liquid phlogiston contain a precise mixture of oil, water and phlogiston gas. Liquid phlogiston allows smaller boilers to burn at tremendous temperatures and pressures that enable portable tinker technology and gigantic vehicles to operate. Horde demolishers use a phlogiston-powered boiler to fling its projectiles.
Liquid phlogiston — odorless, weightless, and almost undetectable — makes steam technology practical by speeding combustion and increasing the heat produced by steam boilers. In addition to its application in technological devices, phlogiston has found use in elixirs that alter the imbiber’s abilities for a short time. Many technological devices use phlogiston as fuel.
The first samples of phlogiston were collected in labs of dwarf tinkers in Khaz Modan when a vacuum pump was used to collect super-heated air over a kettle of molten iron. In the years since, the process has been much refined leading to the development of the phlogiston collector.