The Ninth Legion gets it's name from a band of Roman soldiers in around 500ad. This was the time in which dragonbloods were most volotile due to massive changes among them (Namely discovering they could become dragons), so they were seen as destructive and inhuman by most. These soldiers decided not to withdraw from Britain and instead formed a small army to eradicate dragonbloods. They named themselves the Ninth Legion after the very first documented dragon hunt led my Romans one thousand years before, which they found through a book they siezed.
The Legion heavily documented the period in which they were active, in the hopes the group would keep going for years to come. Unfortunately for them it dispanded 80 years later. The books they wrote in were kept among the former members and were later discovered in a vault by an ex army officer after WW1.
This officer, James O, gathered those of his former soldiers who were willing and trained them in the ways the Romans taught through the books. Over the years, they gained more allies and began their crusade to continue the Romans work. They succeeded and soon were met with an absence of dragonbloods as they learned to go into hiding.
After WW2, more people joined the Legion, some of whom were assassins. Soon after, the Legion expanded to worldwide status with bases all around the world, as small groups of assassins made it their duty to educate new soldiers and rid the world of the dragons.