Sacred Promise

CCS-class Covenant Battle Cruiser. The events surrounding the fate of the Sacred Promise are unknown both to the other characters of Halo and the fans of the game alike. A message from the Sacred Promise was included in the Conversations from the Universe booklet, which came with the extended edition of Halo 2. It is worth noting that a few Transmissions came from the Sacred Promise.

The Black box from the vessel was recovered, though the booklet never specifically mentions where. At the beginning of the recording, the vessel has already sustained heavy damage. The Captain orders most of the weapons of the ship offline due to some kind of extensive danger. He then requests protection from the Devotion and fighter support from the Commitment and Patience. However, the enemy contact fires a "projectile" that ruptures the fuel cells of Commitment and Patience, and it vaporizes almost instantly. There is some kind of impact and the Sacred Promise loses drive control, and descends toward the "Primary satellite". It seems that its own fuel cells were ruptured as well though. There is no other information provided about the vessel, whether any crew survived, or what they were fighting.

The "Projectile" could possibly be a human MAC gun round, but this cannot be confirmed. It could be theorized that it's part of the fleet battling UNSC forces above Earth or Mars (there's mention of primary satellite, indicating the planet might have 2 or more moons, such as Mars' Phobos and Deimos) during the first levels of Halo 2, as it is one if the few instances where UNSC has had relative space superiority through superior numbers.