Adrasta

Lady Adrasta was the ruler of Chloris.

Early reign
Adrasta had men fiercely guard the entrance to Chloris's only mine, granting her a monopoly of Chloris's metal and gaining her power. She was aided in this by Karela and the Huntsman, the latter of whom controlled the Wolfweeds that she grew in her palace nurseries for her use.

Erato came as an ambassador from Tythonus to negotiate the trade of chlorophyll for metal. Rather than lose her monopoly, Lady Adrasta had him thrown into a pit and kept the secret of his offer and identity from her people. She had anybody who opposed her or failed her thrown into the pit to be eaten by "the Creature".

Adrasta tasked her engineers, including Tollund and Doran, with finding out the function of the Tythonian ship. (TV: The Creature from the Pit)

Downfall
Fifteen years after Erato's arrival, Lady Adrasta met the Fourth Doctor and had him inspect the Tythonian ship. She was interested in his theory that it was an egg and had Doran thrown into the pit for providing her with an incorrect assertion on its nature. Hoping that Romana might know something about the ship, she took her prisoner after the Doctor jumped into the pit. She also had K9 brought to her palace, as he was made of valuable metal.

From K9's data banks, Adrasta learnt about the TARDIS and saw the potential in using it as a way to dominate Chloris without using Erato to protect her mine. To this end, she decided to have K9 kill the creature, but this proved fruitless. They came across the Doctor in the pit, whom she also took prisoner. When Erato approached, Adrasta cowered in fear and tried to flee, but the Doctor held her and the Huntsman used the Wolfweeds to force her to use Erato's communicator. Erato told them the truth and, in revenge, attacked her, as did the Wolfweeds. She was killed in seconds and her body dissolved. (TV: The Creature from the Pit)

Behind the scenes
David Fisher named the character partly in homage to Andromeda, a figure in Greek legend who attracted a sea monster which plagued her homeland.