In the Halo games The Forerunners are an ancient race of highly technologically advanced beings. They are responsible for creating the 12 halo rings, the ark, the shield worlds and many artifacts found on planets inhabited by humans. The Forerunner civilization was following in the footsteps of an even more ancient race known as Precursors (their creators) but actually went exiled roughly 100,000 years ago when their war with the parasitic lifeforms known as The Flood culminated in them being betrayed by their own AI, Mendicant Bias, who had been influenced by The Gravemind, leader of the Flood, to take down the final line of defense between them and The Flood. As a last resort the IsoDidact activated the Halo rings destroying the Flood, however the species they had placed on the installation known as the ark survived. This resulted in the survivors (especially the Forerunners) to exile.
Connections to Humanity
The Forerunners were part of an ancient war with humanity. Around 110,000 BCE, the Forerunners went to war with a Pre-historic Human empire, in response to a what they thought of as invasions of new colony worlds. The humans teamed up with the San Shyumm (Prophets) lead by Forthencho. The invasion was perceived to be brought on by human resentment towards Forerunner expansionism during the previous fifty years and the incredible growth of the human population. After the war it was discovered that the true cause of the invasion was the desperate migration of humans to escape the flood. Desperate for new, uninfected worlds to replace the ones lost to the flood, they started to invade worlds of other civilizations, bringing them into direct contact with the Forerunners. The Forerunners perceived humanity's aggression as a threat, and waged a decisive war against the humans for over a millennium, wiping them clean from history, devolving them and sending them back to Earth.
Technological Achievement Tiers
The Forerunner judged the other species by this "Skill tree so to speak". There were 8 tiers, with Tier 7 being the lowest and Tier 0 being the highest
Tier 7: Pre industrial
Tier 7 is the most basic stage of a civilization/Specie's evolution as there is no one united society that is conscious of each other’s activities. Rather they live in a very primitive hunter-gatherer stage society with weapons limited to crude swords and bows and arrows. Technology is virtually non-existent as the only tools that are present as very basic tools. Communication is very primitive and therefore very slow and thus knowledge is very slow to get around. However, rudimentary understanding of the world they live in and the celestial bodies around them is not uncommon in such societies.
Tier 6: Industrial Age
This stage of society is very important to the civilization as their future course is decided. The rapidly expanding population combined with the lack of food demand that new inventions be made so as to continue ones way of life. This is when the first true machinery are invented and the political and cultural identities of a society are formed. The rapid increase in machinery and people demands that medical technology also progress at the same speed, failure to do so will usually result in a catastrophic calamity for that particular society.
Tier 5: Atomic Age
At this stage in a civilizations development, atomic power is discovered. Some species use it for peaceful means while others take the opportunity to launch nuclear war. More often than not these species go extinct. This is also the time period when the first rudimentary space craft are developed.After the turmoil of the Industrial age, things settle down as cheaper and more clean energy sources are discovered and used.
Tier 4: Space Age
At this stage of development, the species begins it first foray into the unknown. Overcrowding threatens to push the society into oblivion and new planets are actively sought after. The species also becomes more and more aware of its surrounding and are more aware of the effects of pollution. Curing hunger and diseases take an increasing priority over other matters. Present day Humans are at this stage.
Tier 3: Space-Faring
At this stage of the game, the society can navigate the stars using Slipspace/hyper speed and have the know hows to build semi-sentient life; the A.I. Humanity during the events of the Halo saga are at this level
Tier 2: Interstellar
They have an almost completely mastery over the integral functions of Slipspace and as such can perform precise jumps in Slipspace. They are also able to use energy fields for protection. The covenant stand at this level
Tier 1: World Builders
Tier 1 is the highest known level that has been reached by a civilization, namely the forerunners. At this stage, travel through Slipspace is completely mastered. They are also able to manipulate gravitation fields and construct huge feats of engineering marvels; namely the Halo rings. Using their technology they are able to cultivate other lives and can even create new worlds.
Tier 0: Trans-sentient
Tier 0 is the Cap of the evolution of species, the Forerunner viewed a Tier 0 society as a society that had technology that surpassed theirs. At this level, they could traverse galaxies and have almost an unlimited expertise in the fields of Slipspace and the manipulation of the Quantum filaments that govern it. The civilization is able to completely manipulate the evolution of life down to a single atom. The forerunners theorize that the precursors might have achieved this state.