iirc (in some interpretation) the last treasure of 7 Treasures is carnelian or something red, which previously I believe the piece Padparadscha bought back from the moon (he/she called it 'heart') will be such part for Phos. >>> foreshadow the madness of Phos, now to become full we have to restore that broken heart of Phos (poor poor Phos)
But, now I realized that Cinnabar hair is also red and just look at how Phos still remember he/she quite well despite losing over half of his/her body portion, I think Cinnabar is the heart.
I don't know what will happen but the only remaining original part of Phos is torso, maybe we will have to say goodbye to Phos here as Cinnabar become the heart or we will meet a new entity.