The Order

The Order was a secretive group of humans left behind by the Forerunners to be the guardians of the Milky Way galaxy. They were charged to protect all life in the galaxy from external threats that the Forerunners believed might be a danger in their absence. In effect, they are the custodians of the Forerunner's legacy. They are sworn to never interfere and to only keep watch, and to protect the galaxy from unknown enemies that might show themselves in the future. As a result, they are seldom seen and rarely revealed to the galaxy at large, using their advanced technology gifted to them by the Forerunners to remain concealed in watching. Other factions, most notably the Royal Allegiance, have limited knowledge of the Order's existence, but nothing of their purpose or anything deeper than that. The Order are the most technologically advanced race in the galaxy after the Forerunner's demise, though they almost never reveal themselves and simply wait for the time when they will be needed. 
==Creation==
The Order did not form during the Forerunner's time. but rather near the ending of the Forerunners. As the Forerunner-Flood war became a foregone conclusion, the Forerunners were forced to come up with a contingency plan. Knowing that the Flood was unstoppable, they gathered all sentient life in the galaxy to the Ark. While doing so they formed the Order, the purpose of which was to protect the now-unguarded sentient species of the galaxy. The Forerunners took a group of humans, genetically engineering them to become the perfect warriors. The batch was put into cryogenic sleep and stored away on the Ark. About 25,000 years after the Battle of the Maignot Sphere, and the elimination of all sentient species not within the Ark or the Shield worlds, Forerunner ships were jettisoned from the Ark, bound for the original worlds of the protected species. 
The Royal Allegiance was created when a Forerunner vessel containing the genetically enhanced humans crashed on the planet Thera. 
The Order, equipped with specially designed vessels of Forerunner origin, began their guardianship of the galaxy, destined to watch and wait for battle to come to them. 
== Technology ==
The Order's technology, like that of the Allegiance, stems directly from the technology of the Forerunners. However, where the Allegiance's technology was reverse-engineered to provide a wealthy base for future technological advancements, the Order's technology comes from an unbroken line directly from the Forerunners- their ships, weapons and technology was made for them by the Forerunners, along with the provisions necessary for the Order to independently grow, develop and advance without the presence of the now-extinct Forerunners. As a result, the Order is much, much more advanced than the Allegiance, the UNSC or the Covenant.
== Involvement with the Galaxy ==
The Order were sworn never to interrupt with galactic affairs, including wars where one species may be wiped out. They were created by the Forerunners to watch over the galaxy's races as a whole, not an individual race. So, when the Human-Covenant War began, and it became clear that the Covenant could not be talked out of the war, the Council of XII came to a desicion that their purpose of guardians would be best served if they allowed the Covenant to destroy the humans. It was one of those cases where the right decision was not the easiest one. The Order did not know of the Covenant's plans to light the rings, if they had, then they would have stepped in. 
The UNSC had never come across any evidence for the existance of the Order. The Covenant suspected another, shadowy faction in the galaxy but had no proof other than sensor echoes.
The Allegiance was aware of the Order's existance. They attempted to make contact numerous times but were ignored every time. One lone Order ship was discovered, once, near the Camulos system, in 2441. It was fired upon by six Allegiance vessels. Reports from survivors were sketchy but seem to show that the fire did not impact upon the ship, it merely passed through it. The ship faded away into the void, leaving no sensor readings whatsoever. It is unknown what occured next, but three days later, a life-raft containing 47 Allegance crewmembers, all unconscious, drifted into orbit of the planet Camulos. Its sensor logs had been wiped clean. None of the crew members recalled what had happened to them. The event was put under wraps by the Allegiance. 
== Religion ==
The Forerunners, knowing they would need something stronger than word to keep the Order from interfering, created a religion for the Order which revolved around the Forerunners being gods, and the Order doing their last will. From the 18th Century Earth time onwards, the religion became more of a choice than doctrine, as it was in the early years. It was no longer required to believe, the Order had advanced enough to realise the religion was a way of making the Order follow the Forerunner's commands. Nevertheless, some, if not most, of the Order's citizens continued to worship the Forerunners as gods and believe in their divine charge to safeguard all of creation.
==Behind the Scenes ==
This species is based loosely off the Peacekeepers from Farscape. They were taken from Earth, genetically enhanced, and used as the peacekeeping force for the massively advanced race which took them from Earth. The advanced race was used as the negotiators while the Peacekeepers were used as the guardians, used under the advanced race's guidance. The advanced race died out and without them, the Peacekeepers did only what they knew how to- they kept the peace, using brutal force.