From Halopedia, the Halo wiki
Reme is the homeworld of the Dazreme, a species in the Covenant fringe.
Reme is an ocean world, almost entirely covered in a planetary sea.
Discovery by the Covenant
Reme was discovered by Covenant relic hunters aboard the Burgeoning Fealty. Home to one of the largest discoveries of Forerunner treasures by the empire, the bulk of the reliquary was restricted to sunken underwater cities inhabited by the aquatic Dazreme, who had repurposed the Forerunner technology in incredible ways to create a thriving civilization. The San'Shyuum managed to broker an alliance with the species, leveraging their unique knowledge of Forerunner technology to ends not fully understood, though the Covenant were unable to penetrate their history or medical science.
As more species were formally integrated into the Covenant over time, the San'Shyuum went to great lengths to keep the existence of the Dazreme and their world a secret, including the practice of executing entire crews that had accompanied a Prophet on a diplomatic trip to the planet. Only a handful knew the coordinates to the planet, those who even knew about the world speaking about it in hushed tones and far away from the prying eyes of the Sangheili. Following the collapse of the Covenant and the scattering of the San'Shyuum, the status of the Dazreme is unclear.