Dorn was born the fifth child of the Mazriln royal family.  He was often  overlooked during his childhood and his parents had no real plan for  his future.  His brothers were being prepared for ruling the family and  distinguishing posts in the military.  Although his education was good  by most standards, Dorn loved the histories of his Dragonborn ancestors  and their storied accounts of epic battles and glory and he neglected  most other lessons.  
During a winter court, a travelling  musician stayed at the family's estate and Dorn quickly became enamored  with the heroic tales the man could tell on his lute.  Almost  immediately, Dorn began learning to play.  Unbeknownst to him, his  father, Zalhan Mazriln, looked down on any of his progeny learning such a  "common" skill and demanded his son to quit.  Openly, Dorn did so, but  continued his lessons in secret.  This continued for several weeks until  a servant discovered a lute hidden in Dorn's room and inadvertently  told Zalhan of it.  Not one to be disobeyed, the Patriarch told his  oldest son to show Dorn a real noble's hobby by having the two face off  in a duel with real weapons.  The oldest son, Yorval, was a highly  regarded fighter and had been groomed since birth to take the family.   He was also a bloodthirsty bully who delighted in tormenting his  opponents.  During the fight, Yorval had cut and bludgeoned the 12 year  old Dorn into submission, taking glee from every ounce of misery his  brother displayed.  Then, instead of calling an end to it, Zalhan  decided to test Yorval of his resolve and loyalty, signalling for him to  kill his brother.  As Yorval raised his sword for the killing stroke,  Dorn used his Dragon breath involuntarily and deeply scarred his  brother.  Before he could be caught and killed, Dorn ran from him  family's home, but to no avail.  He was caught and was about to be  executed when an army Captain by the name of Kairon intervened and saved  him, taking him in and teaching him how to fight.  
As Dorn  grew, he learned and lived nothing but the military life, and learned  how different it was to his beloved childhood stories.  It was better!   Dorn took to it and soon became a small-unit leader at home on campaign  and in the field.  He forgot his old life and embraced his new family  with the 4th Army's Fire Reavers Brigade.  This continued for many years  until the grueling war with the King's Army, when the Fire Reavers were  thought wiped out in a terrible rearguard against the advancing forces  of the King.  After the battle, the Dragonborn families surrendered and  their armies, what remained of them, were taken into the King's.  Dorn,  dealing with his loss of another family, travels with the army, looking  to bury his sorrow in battle.