Vampiric Wyrm
Ancient creatures, vampiric wyrms are ultra rare creatures created from corrupted dragons and pockets of Underdark black magic. The creatures have lost their front legs with the wings adjusting to fill this place. Smaller than a dragon, yet still strong, powerful and deadly.
Vampires of the Vampire Nation hunts these rare creatures out for three reasons: food & body parts, experimentation & breeding, and for riding beasts. The latter is the main reason the vampires hunt them. To turn one into a riding beast is seen as a mark of prestige, and most of the eldest of Elders will have at least one of not two, usually one for general use, and one for military & domination purposes.
It takes a strong vampire to dominate a vampiric wyrm, and many die before they can dominate one. Each must be hunted down, trapped and then the vampire must use their hypnotic gaze on the wyrm. This can take days to weeks to complete, and both can easily be killed or starve to death during this time. Wyrms tend to be strong willed apex predators on a par with vampires, so this can easily explain the reason for the time it takes. Most vampires usually take a retinue to help with the hunt and the trapping of the wyrm, and in the odd case as a source of food, at least for the vampire. A well-fed wyrm makes it harder to dominate so partially starving one makes it easier in the long run.
They usually trap the wyrm in a place with sufficient room for either to move, and room enough for the vampire & his retinue to escape and hide if need be. If not, then the wyrm may hunt down and kill them all.
When dominated, the wyrm will attach itself to it's new master, and specified individuals. The vampire master will be able to be around the wyrm with little bother or potential danger. It will fight to the death for it's new master, and will be taught either as a general riding beast, or to be aggressive against specific opponents or in specific situations. A few retainers are taught to help look after the wyrm as this is seen as below the vampire master's station. They will clean, feed and carry out housekeeping for the wyrm, and teach it specific skills it will need while the master will go more in-depth with this to bond further with the wyrm.
The wyrm will have it's own wing in the master's lair, and usually somewhere high up off the ground to allow it to get airborne easier. It will be heavily protected as the wyrm is now a prized asset and the envy of others. A rare few of the oldest vampire Elders will have more than two and a "small" eyrie of these creatures to show their dominance and power.
Where possible, the vampire master will ride one of these wyrms into combat or to deal with other empires to impress or terrify them to gain the upper hand in war or peaceful negotiations. It can help that the wyrm will happily eat animals like horses and other creatures important to some cultures like the Slaves, Turkic peoples and the Frankish Empire.
The eyrie will always have space to train and groom the wyrm, and to exercise it on a daily basis. Food is stored nearby and sometimes hunting for it is practised to hone the wyrm's hunting instincts into hunting specific enemies. Animal handlers are always present, along with specific guards trained in calming down the wyrm and preventing it escape, while preventing others getting to the wyrm. At the same time, they are also trained to kill the wyrm if it becomes too dangerous or suffers illness / injury that will lead to a slow death.
Propulsion
Obviously the wyrm uses wings as it's primary source of propulsion, and will use it's hind legs when necessary. The wyrm prefers to be on the wing tan on the ground where it has better overall control of movement.
Weapons & Armament
The wyrm has a large powerful bite with sharpened teeth to make for a terrifying attack. This it uses as it's primary attack method. Claws are present that it uses to rend & tear, or hold on with an iron grip. Lastly, some of the oldest wyrms have a breath attack. Nowhere as powerful as dragon's breath it is enough to harm enemies close to the wyrm itself.
Armor and defense
The wyrm has a long, lithe body with scales that helps protect it from general harm, and the vampire master regularly has leather and/or metal armour pieces made with the saddle. These are placed to protect vital areas of the wyrm's body without encumbering it, or stopping it from flying effectively.
Communication Tools & Systems
Shrieks, and squeals are usual vocalisations along with body language is the usual forms of communications. The vampire master will develop specific sounds & methods unique to their wyrm so no-one else can ride it.
"I've seen one once. A vampire rider on a vampiric wyrm is terrifying to behold. It also distracts us from all possible dangers from their kind on ground when one appears. Worse if you see many coming your way.
It was in the deep desert in Persia that I saw that one. An outrider, looking for something. We thought it was looking for us but no. We were near a populated settlement at an oasis, and they were looking to wipe it out. Turned out it was too close to an entryway into a Pureblood Clan territory and they didn't want this.
Wiped out the whole settlement, every man, woman and child. Just the one vampiric wyrm rider, that's it. Never want to see one again."
Azim the Explorer, in the Abbasid Caliphate.
I love your use of a creature as a vehicle.
Gotta shake things up and terrify my players even more haha. It's a wonder others haven't tbh. And a big thanks of course.