- last post: 01.01.0001 12:00 AM PDT
The Forerunner disappeared from existence about 100,000 years ago, but the relics of their existence still remain. The Covenant worship the Forerunner as gods in hopes of finding the same "salvation" they had. It is believed that this "salvation" refers to the Flood. At one point in time, the Flood are either discovered or created by the Forerunner. They contained and studied it within several Halo Installations. To guard access to the Flood, the Forerunner built and left several Monitors and Sentinels to guard and contain the Flood.
Then, they mysteriously disappeared, never to be seen again. The proof of their existence and advanced technology lies in the structures of the Halos, the many artifacts collected by the Covenant, and the supposed Ark on Earth, the key to activate all the Halos. It has been speculated that the Forerunner may actually be the direct ancestor of both the Prophets and Humans. The Prophets claim they evolved from an abandoned planet of the Forerunner, and their insignia are found throughout the Covenant structures and uniforms. For Humans, the Monitors are unusually familiar with mankind, and the idea of the Ark being on Earth is quite a coincidence.
Flood
The Flood is a species with virus-like behavior. They're only goal is survival, and survival is only achieved by infecting the bodies of a living organism of sufficient biomass. The Flood infect life forms by attaching fairly large parasites and injecting their genetic material. The virus spreads throughout the body focusing on the nervous system. After overcoming the nervous system, the Flood uses the body for its own purposes. Ironically, within the body of an organism, it has great control and a form of "intelligence". This can be seen as Flood forms are able to utilize the host's technology.
Monitors and Sentinels
Monitors (343 Guilty Spark and 2401 Penitent Tangent) are the caretakers of the Halo Installation and guardians of the Flood. They are a form of AI and were created by the Forerunner in order to ensure the Flood are not released throughout the galaxy. If the Flood breaches containment (as done by the Covenant at Installation 04), then the Monitors' protocol forces them to activate the ring which will destroy all sentient life (Humans, Covenant, or anything) to prevent the spread of the virus. However, Monitors are inhibited to activate the rings themselves. Instead, they must use a "Reclaimer" such as Master Chief or any other human to accomplish that task.