Rubedo is the final stage of the process of creating the philosopher's stone in alchemy, following nigredo, albedo, and in some texts, citrinitas. It means "reddening" and is often symbolized with a phoenix. In Jungian psychology, rubedo often represents the culmination of the whole and the development of the full "self".
Design
Rubedo's appearance as a flaming bird is in reference to the alchemical symbolism of the phoenix.