Because of the Infected Monsters, traversing on the ground had become difficult. This forced humanity to find another means of transportation. Since the Flaming People had affinity to fire, it was easy for them to fight and tame dragons. One for the first time, two next, then a horde of them until dragons became like houseflies in their realms.
The ones used in the transportation field were labeled Floating Fleet, their bodies under a spell that weakened and restrained them from breathing fire. Simply, they were just porters, not fighters.
It was even these dragons that supplied the Drumline Castle with daily necessities, returning to Flaming Realms with porcelains and many other products made by the artisans at the castle.
They were just a Floating Fleet.
Harmless.
