Hadyn has been around for thousands of years, having witnessed the war of the first dragons, though he neglected to fight alongside either any the dragons or the humans who assisted them. Once the war came to an eventual end, Anankos, alongside the humans who he had formed bonds with during the war, created a land known as Valla, a great secret to the kingdoms of Hoshido and Nohr, with only the royal families of each trusted with the knowledge of it’s existence. In other words, it was a land of peace, somewhere quiet where Hadyn would not plunged into war. This prompted his decision to leave behind the newly formed nations of Hoshido and Nohr, and to reside within Valla.
Here, he eventually became friends with the Vallite royal family, as well as Anankos, respected by both as the second most powerful being in Valla, second to Anankos, of course.
Yet, he was not able to assist the Vallite people as the dragon began to degenerate, eventually overthrowing the king and taking the throne, turning Valla into a land of the dead. The phoenix had found himself clashing with one of the early Kings of Valla, whom Anankos treasured greatly, like his own children. This led to Hadyn being sealed away within the kingdom of Valla, away from the Vallite people, somewhere where he could not be stumbled upon and the seal lifted.
The mythical creature who had had once been respected by the Vallite monarchy had vanished suddenly, his disappearance unexplained to the people. Rumours of his whereabouts varied, though none accurate. It appeared as though Hadyn would be sealed away for eternity...
That is, until, Corrin stumbled upon him. Azura, who had suspicions on where he would be found, based on the patterns of her family’s previous punishments, suggested that he would be a great asset to Corrin’s army, leading to the decision to explore this very location. This seal could only be lifted by one of Anankos’ own bloodline, and it was.
However, his awakening was certainly not one of welcoming. The phoenix was, outraged, to say the least, of his punishment and released this rage on Corrin, as the first person he saw after being freed. Corrin was able to defeat the Phoenix, who was then calmed down by Azura and the pendant, to listen to reason. Hadyn explained that he had been sealed away, and thanked Corrin for freeing him. Corrin and Azura persuaded Hadyn to join their cause: To dethrone Anankos and restore the kingdom of Valla, ultimately bringing peace to all three kingdoms.