A Forerunner Super-weapon
At the beginning there were 12 Arrays (rings), they would be scattered around the Milky Way galaxy in order to wipe out all the sentient life from the galaxy. This was needed because of the Flood, who were infesting all life in the galaxy, and apparently could only be stopped by getting rid of its food. The 12 rings were made using both the Lesser and Greater Ark and were subsequently spread out over the Solar System in a previously determined pattern, in order to achieve ultimate devastation.
Mendicant Bias' Rampancy
Once created, the Forerunner Master Builder Faber wanted to test the ring's capabilities. He tasked 032 Mendicant Bias with conducting the first test of a Halo Installation near Charum Hakkor. However, the firing of Installation 07 released an ancient being known as the The Primordial; the entity was taken on the installation by Faber's who assigned Mendicant Bias to interrogate it. During this conversation (43 years), the Primordial managed to convince Mendicant Bias to turn to rampancy and defect to the Flood. He started with using 5 Halos to attack Maethrillian. The remaining 7 were sent back to the Ark in order to finish them.
The First Firing
In order to halt the Halo's work, Mendicant Bias was searching for the Ark. Mendicant Bias was aware of the existence of the Arks, but did not know its exact position. However, he knew that the way to the Ark's lay with The Artifact or the Forerunner Keyships. Mendicant Bias then attacked with a fleet of over five million ships and completely obliterated the Maginot Sphere, and forced the Forerunners to accelerate work on the Halo Array. In order to give them time to complete the Halos' work, the Forerunners created another AI, Offensive Bias, to slow Mendicant Bias' advance. Even though Mendicant Bias' Fleet outnumbered Offensive Bias' by 2183:5, Offensive Bias was designed specifically to effectively counter and exploit Mendicant's rampant actions. In order to defeat Mendicant Bias, Iso-Didact began the activation sequence from the Lesser Ark, succesfully. Those saved were sheltered on the Ark. What remained of the Forerunners on Installation 00 then left the Orion arm of the galaxy, the Flood were contained, and the centralised Gravemind form eventually died. Then, Keyships began escorting various alien life forms back to their respective homeworlds, so that life could blossom across the galaxy once again, with no threat of the Flood.
Rediscovery Of The Halo Arrays
The seven Halos then remained relatively dormant for one hundred thousand years until Installation 04 was discovered by the theocratic alien superpower, the Covenant. The Covenant, who revered the Forerunner as gods, believed that the Halo's main weapon was actually a source of "Divine Wind" meant to propel them on a path they called the 'Great Journey'. Here, the Covenant unwittingly set free some Flood spores, which were unsuccessfully fought. Then, when a human ship appeared, the battle provided the Flood with more than enough bodies to infest. In order to halt the Flood's progress, the Installation's Monitor tried to use a Reclaimer to active it, and thus destroying all nearby sentient life. This plan was halted however, when a Human AI discovered the truth behind the Installation. This prompted the human to destroy the ring in an alternative way. Shortly after the destruction of Installation 04, the Lesser Ark started working on a replacement, which would be placed nearby the remnants of the destroyed ring. This would provide the Halo Network to have a galaxy-wide effect when fired.
The War Goes On
The Human-Covenant war was not done, after the destruction of Installation 04, it eventually moved to Installation 05. Here, the Flood had escaped and was again threatening members of the human and Covenant armies. In order to fulfill the task to start the 'Great Journey', the Jiralhanae Chieftain Tartarus kidnapped two humans in order to ignite the ring. This plan was stopped however, but not before the ring was ready to fire. Each Halo Array has a security in it that will force all other rings to go into standby-mode when a sequence is halted. This means that now all the rings could be ignited from the Lesser Ark.
This meant that the war turned towards the Lesser Ark, and thus towards the Portal to it. Once on the Ark, the Covenant's leader had kidnapped a human and was going to ignite all the Rings. He was stopped by a short-lived alliance between the humans and the Flood, who both did not want the Rings to be fired. Once the sequence had been stopped, the replacement for Installation 04 was to be prematurely fired. Since it had not been fully completed, this would mean that the nearby Ark would at least suffer great damage. This would mean that the Ark no longer could be used to remotely activated the remaining Rings.
The Remaining Rings
Even though the Flood were seemingly destroyed, the Rings were still there. The humans took it upon themselves to find all the Rings, decommission them and learn all their secrets. Not much is known about the other Halo's, but this may change in the future. A especially large portion of the destroyed Installation 04 was later discovered, infested with Lekgolo