Astra of Atrios

Princess Astra of Atrios was the sixth segment of the Key to Time.

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As the Princess, she was the sixth child of the sixth dynasty of the sixth royal house of Atrios. Astra was kidnapped by the Shadow and taken to the Shadow's Planet of Evil. She was later transmuted to her true shape. After the Doctor had defeated the Black Guardian, Princess Astra was restored to her humanoid form and returned to normal life, ruling her planet and presumably marrying Merak. (DW: The Armageddon Factor)

Soon after Astra was restored, the Doctor's companion, Romana, regenerated and chose as her final form the appearance of Astra, for reasons unknown. (DW: Destiny of the Daleks)

Without the energy of the segment to keep her together, Astra's was 'passed on' to Romana after the latter's regeneration. Astra has been unintentionally draining the life-force of her subject Atrians. She then went on a quest ofr the Chaos Pool in the belief that it may save her people. In an encounter with the the Doctor, Romana, and the Doctor's new companion Amy. Astra did not recognize the Doctor due to her memory of her meeting with him having been erased after her restoration. Astra allowed herself to become the sixth segment once again to save her people from her actions. The Doctor then destroyed the Key in the Chaos Pool to prevent either Guardian from claiming it. (BFA: The Key 2 Time - The Chaos Pool)