High Dragons

High Dragons are the most successful of the Ancient Dragons attempts to prolong their own existence - yet still a failure. They were created the closest that the ancient dragons came to replicate their own species through arcane means, but still too divergent (and weak) to maintain their souls. But like many of the creatures created as a part of that project, they were eventually left to fend for themselves in the wild.
The high dragons persevered - and spread. Today there are several thousand of them, each of them a singularity of pure power requiring small armies of normal mortals to engage them in combat (or a small group of truly overpowered species, like the High Elves, alternatively a demigod or a Chosen One) to have even the slightest chance of taking them down. They have conquered their way into undisputed rulership of the Kingdom of Drakha and are still encountered beyond its borders.
This is derived mostly from their mentality. Every single high dragons possess and intense desire to be on the top, to control others and to own - be it gold, slaves or land. This desire can be occasionally suppressed or be achieved through morally acceptable means, but most high dragons (due to their inborn power) rarely bother with anything other than conquest or pillage. There are high dragons that has managed to left their mentality behind through conscious actions, but they are a small minority.
They are also shapeshifters, ones considered a very magical species - enough to be capable of successfuly crossbreeding with about anything that moves (although they tend to limit themselves to other mortals/daemons, and only in their humanoid form, since nothing good comes from different pairings). Mostly by the virtue of such pairings working less on the basis of biology and more on magic.

Basic Information

Anatomy

High dragons are a heavily magical specie, naturally capable of polymorphing between several forms - one humanoid, one dragon, and several in-between, mostly humanoids with various dragon additions, for example scales, horns, sharp teeth and so on.
It is heavily depended on their level of blood purity, with very pure dragons being capable of merely their humanoid and dragon forms, without those in the middle. Their human form is permanent (which means that while it undergoes changes due to age, it is impossible to change via polymorphy into others). Their dragon form is, too, permanent, with all scars and so on remaining forever.
In their dragon form they look similar to their progenitors, Ancient Dragons. They are large, strong and with three pairs of limbs - rear legs, front legs and wings. They, which is also similar to ancient dragons, are capable of flight due to passive ability to lower their own mass.

Genetics and Reproduction

Their genetics are heavily dependant on their level of blood purity, over time risking genetic degradation into lesser orgreater dragons, who are a species of beasts - so by all means purposes, animals. Their level of blood purity also defines, how well they manage to polymorph. Oddly enough, the less pure their blood is, the stronger they are.
This is caused by the fact that their non-dragon part is directly connected to their sentience and magic. The less they breed with members of the other races - and more with their own - the more they degenerate, with every sucessive generation much weaker and more bestial. Finally they end up without possibility to assume humanoid form, being nothing more than slightly more inteligent lower dragons, mating with them and finally losing any sort of significance.
Their fertility is extremely low, with average high dragon mating regularly with 4-5 partners (they tend to have polygamy centered on themself) producing one offspring every 30-40 years at best. On the other hand, they are capable of reproduction practically until death, and they can breed with next to every other race that does reproduce in a way similar to humans. The species whose form they achieve after morphing into humanoids depends on their non-high dragons progenitors.
They reproduce in a relatively typical way. Impregnated female high dragon temporarily lose their ability to polymorph, which is actually typical for polymorph reproduction, since it is the easiest way to avoid somehow 'damaging' the child in the process. In case of male high dragon impregnating female from another race, the pregnancy is noticably longer, taking up to 15 months.
They are birthed in human form, soon however morphing into dragon form, which is much tougher, significantly increasing survivability during infancy while avoiding the trauma of human woman giving birth to little dragon.
They learn to control their form and change it into human with time, depending on purity of their blood. If they won't learn it until their 20 birthday, they are considered lesser/greater dragons and - in most cases - killed.
Very pureblooded high dragons tends to spend majority of time being dragons, while being seriously... animalistic. When that happens they tend to be dangerous, attacking travellers. Especially those of opposite sex are in danger, and sometimes a type of genetically imprinted failsafe fires up, making them feel intense lust for potential partners that will prevent his genes from further devolving.
This mechanism is the source before the legends about offering dragons virgins, to - quite literally - 'sate' their rage. Thankfully, the very strict laws in the high dragons communities has essentially eliminated this problem in modern times, since a high dragon degenerating so much is almost unheard of.

Growth Rate & Stages

Until ~20 yo they are considered infants, still in their dragon form. After learning how to polymorph they are considered children, with their humanoid body looking accordingly, and their sexuality yet to awaken. When their dragon form stops growing (around ~100 birthday) they are finally considered adults, while also undergoing a seriously late puberty in their humanoid form. They die from old age around 500 birthday.

Ecology and Habitats

High dragons are naturally domineering creatures, that feel the best when they rule over others. They also enjoy gathering treasures, which they consider a symbol of their power (just as artifacts, trophies and so on). Due to this, many tend to live in 'lairs' separated from civilization, with small group of high dragons (in most cases only one +offspring too young to live on its own) lives.
With a group of (often slave) concubines, and group of servants and/or subjects, with servants and concubines giving them sense of power, while forests and wastes surrounding their 'lair' (often formerly abandoned castles and similar places) treated as their personal playgrounds, when they can hunt in their dragon form. This can be considered to be a habitat of a notably more 'civilized' high dragons that has mostly kept themselves under control.
The less moral high dragons will typically end up attempting to conquer whatever they are, bending their knees only before something that has proved to be massively more powerful than themselves. The result are the societies similar to the Kingdom of Drakha, a hegemony of a single most powerful high dragon followed by few thousand of his underlings, each of them having an undisputed control over some region of Drakha - and often seeking to expand their domain through warfare.

Dietary Needs and Habits

An interesting thing about them is that their level of satiation varies between their dragon and humanoid forms. While being sated in humanoid form has no direct impact on their dragon form, they DO tend to slowly be sated 'offscreen', requiring weeks of being well fed for some serious improvement - and vice versa.
As big creatures (and very magical one) they tend to enjoy food with high ammount of callories, mainly sweets, meat, beer and so on. On contrary, their requirement for vegetables and herbs is very low, and exists only in their humanoid forms.

Additional Information

Uses, Products & Exploitation

Various parts of their body are found interesting from the alchemical point of view, in most cases completely different depending if they came from their humanoid and dragon forms, with their middle forms being sort of a mix between the two.
Their scales, teeth and claws are also quite costly crafting materials, used in armors, swords and arrowheads.
The supply of such resources is severely limited by the fact that attacking typical, 'civilized' high dragon is still considered murder, with (almost) all resources on market coming from high dragons that has overstepped the boundaries of the law and had adventurers set after them.

Perception and Sensory Capabilities

Eyes and hearing extraordinary, also possesing ability to detect heatsources similar to snakes, at least in their dragon form. For some reason they did not inherit ancient dragons ability to feel magic.