Here is what to know about WoW dragon lore heading into Dragonflight.
The evil dragon broods like Nefarian were attempting to create a chromatic dragonflight as their weapons to rule Azeroth. Chromatic dragons are incredibly powerful, nigh unstoppable. But it is so difficult to create them and requires so much power that they kept failing (luckily for us). At one point, they were trying to infuse an inert chromatic dragon named Chromatus (an incredible beast with 5 heads, each one a different color representing each flight) with massive amounts of power and it almost worked, the creature stirred for a bit but heroes were able to stop the process. However, a chromatic dragon is so powerful that they could not even destroy the body, just lock it away in hopes it never fully wakes.
The rumor is that 10.0 is about Chromatus waking at last. And it is unstoppable. We are playthings to it. Think Ghidorah from Godzilla lore. With all the old dragonflights extinguished or powered down, perhaps our new quest will be to empower a NEW dragonflight that can counter Chromatus.