from the Synaxarion:
The Nobleborn Empress Pulcheria, daughter of the Byzantine emperor Arcadius (395-408), was co-regent and adviser of her brother Theodosius the Younger (408-450). Having received a broad and well-rounded education, she distinguished herself by her wisdom and piety, firmly adhering to the Orthodox teaching of faith. Through her efforts the church of the Most Holy Mother of God at Blachernae was built, and likewise other churches and monasteries. With her assist, the Third Ecumenical Council was convened in the year 431 at Ephesus, to deal with the heresy of Nestorius.
Through the intrigues of enemies and also Eudocia, the wife of the emperor Theodosius the Younger, Saint Pulcheria was stripped of rule. She withdrew into seclusion, where she lived a pious life. But without her things became disorderly, and after a certain while, upon the urgent request of her brother the emperor she returned, and the unrest provoked by emerging heresies was quelled. After the death of Theodosius the Younger, Marcian (450-457) was chosen emperor. Saint Pulcheria again wanted to withdraw into her seclusion, but both the emperor and officials besought her not to forsake the rule, and instead become the spouse of the emperor Marcian. For the common good she consented to become the wife of Marcian on the condition, that she be permitted to preserve her virginity within the marriage. In such manner the imperial spouses lived in purity, like brother and sister.
Through the efforts of Saint Pulcheria, the Fourth Ecumenical Council was convened in the year 451 at Chalcedon, to deal with the heresies of Dioskoros and Eutychius.
Throughout the course of all her life Saint Pulcheria defended the Orthodox teaching of faith against the various heresies that emerged. Having distributed off her substance to the poor and to the Church, she died peacefully at age 54 in the year 453.