The Parliament of Rats was a pirate ship captained and crewed by human mutants on Lethe. It was captained by Pym.
The Parliament attacked the Lung of Heaven while the Fifth Doctor and Nyssa were aboard. The captain of the Lung, Katr Korzen, killed Pym, and her crew drove off the rest of the pirates. The Doctor, on board the Parliament, convinced the White God to kill itself, which destroyed the Parliament. (PROSE: The Parliament of Rats)