From Halopedia, the Halo wiki
Metisette is a natural satellite orbiting the gas giant Hesiod in the 23 Librae system. It has a mostly nitrogen atmosphere, but methane is present as a liquid, forming the rivers and lakes of the moon.
In 2535, the Kig-Yar Reth used Metisette as a breeding grounds for hundreds of thousands of Unggoy, which would be used to seize the Rubble, and, ultimately, to destroy Earth. A structure constructed under the supervision of Reth, known as the Redoubt, was used to help the Unggoy mating process.
UNSC forces landed on Metisette to attack the Covenant soldiers and destroy a Kig-Yar AA battery and a sensor facility, initiating the Battle of Metisette. Reth's plans fell apart when the Unggoy breeding grounds, along with the Redoubt, were destroyed by the asteroids of the Rubble. The Rubble's AI, Juliana, used the Rubble's asteroids to orbitally bombard the surface of the moon to defeat the Kig-Yar and Unggoy.
Non-canon and dubious canon appearances
In 2552, while pointing out the visible worlds in the sky from the Rubble, Kessler showed Kwan Ha Metisette. Kessler told Kwan that no one goes to Metisette and that he didn't think that there was any air there.
List of appearances