From Halopedia, the Halo wiki
Rantu, along with Uhtua and Rentus, is one of three named natural satellites of the planet Te, the homeworld of the Lekgolo. In addition to these three moons, there are twenty-two other satellites in orbit around Te. Rantu was in orbit around Te near the remains of a Forerunner orbital station, known as the "Rings of Te". The moon's atmosphere is unbreathable to Sangheili.
At some point, Rantu was colonized by the Lekgolo. During the Taming of the Lekgolo, the Lekgolo went to war against the Covenant. When no conventional methods could end the war, the Hierarchs of the Covenant sent Arbiter Jeht 'Lcmutee, who journeyed to the moon in a dropship to learn more about the Lekgolo. For an entire year, the Arbiter searched the surface of the moon to learn the strengths and weaknesses of the Lekgolo. On Rantu, the Mgalekgolo were first encountered by the Covenant when a Lekgolo colony combined into several Mgalekgolo forms to battle 'Lcmutee. With the knowledge he gained on Rantu, 'Lcmutee would suggest for the Covenant to incorporate the Lekgolo into the Covenant and peace was eventually made between the two parties.