Erato

Erato was a Tythonian ambassador, he was sent to Chloris to negotiate a trade deal, but instead found himself trapped by Lady Adrasta, who captured him and stole his communication device to prevent any infringement on her trade monopoly.

Biography
Erato was sent to the planet Chloris to try and obtain trading rights between it and his home planet of Tythonus, however, before he could make any contact with the inhabitants he was captured by Lady Adrasta, who stole his communication device and threw him into "The Pit" to maintain her monopoly on the metal in Chloris.

Erato managed to transmit a distress signal which was intercepted by the Doctor and Romana. The Doctor rescued him from the pit, but only to discover that the Tythonians had declared war on Chloris due to the missing ambassador, and had dispatched a neutron star to collide with Chloris's star and destroy the system. It was due to collide within twenty-four hours. Working against the odds, the Doctor used the TARDIS to stabilise the star, while Erato weaved a metal covering around it neutralising the danger.

The Doctor’s last act on Chloris was to push Organon, now one of the de facto rulers, toward a mutually beneficial trade agreement with Erato and the Tythonians. (DW: The Creature from the Pit)

Trivia

 * Erato was the name of the greek muse of lyrical poetry.