A separate post for those who only want the stat block for the cloud dragon.
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This dragon was pretty fun to design! One of the things I liked about this is how the two breath weapons combo with each other and make the environment that much more difficult to fight in. The lingering breath weapon cloud is dangerous to be in for a variety of reasons. Players will have to think more cleverly or make decent preparations to defeat the cloud dragon. Compared to other dragons, the cloud dragon has abysmal AC, which is mitigated by its obscuring breath weapon and high mobility at older ages.
The flavor is decent and sets up some future ideas. I might even sprinkle the rest of the primordial dragons into other compendiums like a cameo or something. I think the idea of affecting an area or altering terrain is going to be a cool characteristic of primordial dragons moving forward, so I look forward to designing those too.