The Vostroyans are one of the oldest Imperial Guard regiments, able to trace their regimental histories back to the Horus Heresy. The Vostroyans have a tradition that the firstborn son of every household will serve in the Vostroyan regiments, hence the name of the Regiments. Vostroyan soldiers and officers retain a very archaic appearance on the real world century Russian Cossacks. Firstborn use long and very ornate rifles, usually with wooden stocks. Officers bear lasguns integrated with axes. Thus, the Vostroyans are probably the regiment most evocative of the "retor-tech" background of Warhamer 40,000.
Vostroya is a world in the Halo Zone beyond the Eye of Terror, which swore allegiance to Mars during the Age of Strife. It is now ruled by a committee of Techtriarchs, a mix of Mechanicus Magos and more traditional Imperial planetary government. During the Horus Heresy, Vostroya refused to provide regiments to the Emperor, preferring instead to reserve the population in the manufactorium blanketing the world. After the Heresy came to an end the Vostroyans agreed to supply every first born son to service with the Imperial Guard.
According to Vostroya's Offering of the First Born, the eldest son of every family must enlist in the Regiments. There are no exceptions to this law, which applies even to the Techtriarchs and the most noble families. For most Vostroyans, it is an honour to join the Firstborn, as great importance is attached to the payment of the debt owed to the Emperor millennia ago. Unlike other Imperial Guard regiments that are raised en-masse, the Firstborn get a steady flow of new recruits from their homeworld. As a result, the Firstborn regiments have existed unbroken since they were first created. This is not always easy to organize, but ensures that Vostroyan regiments maintain their highest potential, as well as ensuring there's a core of veteran warriors. One consequence of this method is that the Firstborn are considered a brotherhood of warriors and are proud to train the recruits according to their own exacting standards. Few Imperial Guard regiments have a stronger brotherly bond than the Vostroyans. The Firstborn are trained in very difficult conditions among the ruined industrial areas of their homeworld.