Why Arkayna’s family guards the Dragon Disk
Given the involvement that the Astromancers have with the Mysticons (it is mentioned in the chapter books that the original mysticons worked with the astromancers) I would have thought that the Dragon Disk would be held at the astromancer academy. There is only one reason I can think of to explain why Arkayna’s family had it, and that’s because Arkayna and Zarya are descendants of Imani Firewing, if not the original Mysticon Ranger as well.
My theory is that Imani’s firewing’s next-of-kin managed to be allowed to protect the Dragon Disk, as it is important in the family. If the original Mysticon Ranger was related to Imani Firewing, just as Zarya is to Arkayna, then it would be even more likely for the family to obtain the Dragon Disk. The Astromancers probably allowed that, either because of sympathy (they knew how hard it must have been to see Imani battle Necrafa) or because they figured anyone stealing it would check the Astromancer Academy, but not a random house (considering that Arkayna’s ancestors took over the kingdom, it may have been possible that her family wasn’t royal at the time).
One potential drawback to this theory is that Imani looks nothing like Arkayna or Zarya, but i don’t think that disproves the theory. Imani was alive 1,000 years before the series began (Em mentions the battle of victory heights being 1,000 years ago). Assuming a generation is 30 years, there would roughly be 33 generations between Imani and Arkayna and Zarya. And considering that only 50% of DNA is passed between each parent to their offspring, the amount of DNA Imani would share with her current descendants are very small, so it makes sense Imani doesn’t resemble them.
Tl;dr: The reason Arkayna’s family has the Dragon Disk is that they’re descendants of Imani Firewing. The lack of resemblance doesn’t matter, as the timespan means that the DNA shared between them would be very small.
