1) Spell check. chieftains, not chieftans.
2) Grammar check. Comma after "After seeing the new design aesthetic in MM3", and other grammatical mistakes.
3) Put in an introductory paragraph, don't just jump right into the description of your adventure.
4) Your adventure description is kind of all over the place, in terms of chronology. It's a little confusing. I'd put it in order for clarity's sake.
5) Your prose is a little too purple

I'd consider toning down sentences such as: "In the cavernous darkness, Vyakti holds the upper hand, for any light draws the ireful gaze of the dead god." You aren't writing a pulp novel, and you don't want to make the DM giggle too much
6) It might be helpful to emphasize the fact that the PCs are working to protect the civilized lands from invasion. They are on a covert mission, but who sent them?
7) It's a bit of a stretch that the bad guy is killing a dragon just to show-off as a prelude to invasion. Perhaps there's another reason he has to kill the dragon? The dragon has a powerful artifact? Or he has to win the loyalty of the divisive dragonborn tribes?
8) I like the three dragon battles as the three "acts" in the adventure. I would emphasize that structure in your description, not just in the title. I would also emphasize how these battles will all be very different.
Good luck!