The 13 'original' Phoenixes vowed long time ago to eternally protect the world from a great evil entity long since forgotten - which they barely managed to imprison with their mix of arcane and sublime prowess - and such was the strength of their vow that it became reality. At one point of time there are exactly 13 Phoenixes - no more, nor less - when one of them dies his spirit reincarnates in another vessel.
Ebon Phoenixes ???