The War of Metamorphos

a lone figure atop a hill, in wizard garb, in the blinding sunset. you cannot make out their features.
Long ago, there was a powerful group of human beings. They called themselves the Magicae Veritates. Their leader was a powerful magic user, renowned across the lands by humans, who took many apprentices under their cloak. They taught their pupils the magic of Stacking. A particular magic that does not give much to the beholder; under the guise of their leader, the students blamed themselfs for their apparent lack of magical prowess. Stacking is a method in which one magical pool expands as it pulls from the reserves of others. Their leader became even stronger, and they made their message clear: "The Creatures' magic is a threat. We, the Magicae Veritates, shall rectify that." As vain creatures themselves, the rest of humanity did not object. And even if they had tried, their magical powers were much too little compared to that of the Magicae.
creatures, forced into human bodies, run from a blinding light. you cannot make out their features.
With the innate magical prowess, and the Stacked magic of dozens of pupils, the Magicae Veritates cast a spell of unprecedented magnitude. It swept across every ocean, mountain, and forest. It dug deep into every cave and high into every nest. This spell was ruminated upon for many years, tested times upon times, only to utter perfection. A modified form of Stacking. It would force the Creatures--- who have a much higher magic pool on average than humans--- to use most, if not all, of their magic reserve on maintaining a human form. This would make them less dangerous, beautify them to the human eye, and force subservience. It was humanity's way of taking the forests for themselves.
a featureless black dragon steps out from smoke.
The Dragon's magic reserves are exponentially high. This was one of the main motivations for the neutering of all Creature's magic. Yet, because of the Dragon's magical potential, they had much more magic left over than intended...
a featureless black dragon readies up an elemental fire attack from its mouth. its red eye is piercing.
The most powerful of them all needed not even conjure a human form. The vanity of the Dragons rivaled that of the human's.
featureless bodies float in a white void. One is human, one is both Creature and human, and one is a Creature..
...Eventually, a compromise was made. The magic of Creatures was still limited, as they still had to tend to a humanoid form. But the more parts they kept from their original bodies, the less the toll. Moreover, they gained the ability to freely transition between forms. All of this took place in the distant past... And now, we Dragons face adversity for the good we once did.