Dracoplague

The Dracoplague is a mysterious disease that haunts all of Dragon kind, reducing them to more beastial creatures of violence and rage, and fundamentally altering their physiology. The disease affected more than just dragons, it also affected Dragonborn, kobolds, half dragons, lizardfolk, drakes, fortails, slyzards, wyverns. The disease spread fast throughout Dragon kind, and brought an end to the High Empire of the Dragons, an end to the domination of Dragon kind, and end to the culture and civilization of the dragons.
The Dracoplague itself is little understood, it is known that the Aen Selath, powerful mages of the Elventine Empire, created the Dracoplague, but it is unknown how they did so, or how they spread it to encompass all of Dragon kind, and vertically bring the Dragon to precipice of extinction.
An excerpt from the rare book The Dying of the Dragons:
‘Once the skies were filled with the intelligent and majestic dragons, their Empire extended throughout all of Runedastil and the heavens above, they took humanoid races under their wings to elevate them, created great works of art, practised the greatest arcane arts, and combated the threats of the savage giant kind. Wars stretched beyond millennia as the dragons staved off the degenerative giants but the dragons were left weak, but those that had benefited from the dragons the most betrayed them, killed their young and shattered their Empire greatly reducing their number. This was not the great affront and crime against dragonkind, that was the Dracoplague, a horrific and ill-conceived magical disease which sought the extermination of dragonkind; it was instead far worse, it devolved dragons into mockeries of themselves, violent, beastial, unintelligent, and contemptuous creatures. The damage was not, however, restricted to dragons, all draconids and draconic humanoid races were impacted in some way, causing harmful changes to their physical or mental make up. While betrayal can be redeemed, an attempt to permanently destroy an intelligent and articulate race is an act of great evil for which the elves can never be forgiven for.’
And from the tome About Dragons:
…but it was truly the Dracoplague the work of the Aen Selath, … that led to their current situation. Today true dragons… are near extinction. Only the lesser dragons exist in any number, drakes, fortails, slyzards, wyverns, and the worst the Dracoplague corrupted dragons, but to compare these creatures to true dragons would be like a caged budgie to a royal griffon. Dragons were brought low by the Dracoplague and by professional hunters from the mortal races.