So, I got this bit of an idea, where it's a fantasy novel world where species of birds are stand-ins for fantasy races based on their cultural interpretations IRL. As an added part contributing to this theme, I decided to make a unique subrace, that any race can become; a Phoenix.
Generally, the idea is that a Phoenix has powerful magical ability from within, their bodies basically being a well of power. They're immortal, and when killed by conventional means, they combust and are reduced to ash, and soon return in their previous form. Killing one for good is like trapping a soul in an Elder Scrolls game, you must enchant your weapon or cast a spell to "steal" their essence.
So, what I was trying to figure out weaknesses for them, and how important they will be in the politics of the world. On one side, most politics are controlled by cabals of Phoenixes who carefully select potential mates and family members to basically "control" the creation of their race (As Phoenixes can be created simply by breeding with other ones, or by chance with one Phoenix and a regular mate, the latter is encouraged since the former would eventually devolve into incest), and not let the world get out of hand with people naturally being able to cast spells. (Regular birds can learn how, but must study and practice how to harness it - a feat if you want to use anything more complex than a simple light spell) The other side actively hunts down Phoenixes to use as a power source towards the physical god they worship, who was a Phoenix himself who ascended to immense, godlike power from the sacrifice of many Phoenixes in his name, akin to the deities of the world who earned their status through similar means, but ascended to a higher plane through a much, much higher body count over a larger amount of time.
One of the major plot points about halfway through is that Phoenixes abruptly stop being immortal, and no longer get resurrected, in the middle of a deadly war. The characters set out to find the reason why, and how to bring back their status, which involves the slow awakening of an eldritch abomination that far outclasses literally everything in sheer magical power, siphoning the world and its people for its own awakening.
So, I'd like your thoughts, what would be good weaknesses and limits for them, at least for the average one? I feel like this can go a bit too far in terms of unintentionally making things a bit convoluted and far-fetched from the existence of people who are basically way too powerful, so I think setting limits would be a good place to start.