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 and the White Guardian had used the Key to restore universal balance, Princess Astra was restored to her human 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. (DW: Destiny of the Daleks)''

Astra returns during the events of The Key 2 Time - The Chaos Pool, where it is revealed that, without the energy of the segment to keep her together- her status as the segment having been 'passed on' to Romana following her regeneration-, Astra has been unintentionally draining the life-force of her fellow Atrians, forcing her into a serach for the Chaos Pool in the belief that it may save her people. In an encounter with the Fifth Doctor, Romana, and the Doctor's new companion Amy- Astra not recognising the Doctor due to her memory of her meeting with him having been erased after her restoration-, Astra allows herself to become the sixth segment once again to save her people from her actions, the Doctor subsequently destroying the Key in the Chaos Pool to prevent either Guardian from claiming it.