Forerunners are most likely, the first lifeforms to appear in the universe after the Big Bang. They only possessed a low-level intelligence level, enough for them to seek out food and consume it. Their food was any form of matter in the universe. Although they were unable to consume stars, they used starlight to guide them and flew to the stars like moths to a flame
The first forerunner appeared billions of years ago, in a dwarf galaxy far from the Milky Way. It was a lifeform made of powerful magnetic fields that developed around a gas giant. Once it attained sentience, it realized that eventually its homeworld would die and it needed to be able to leave it to survive. It created much smaller artificial lifeforms, consisting of the same form of EM field as his. To animate and program its "children", it created the first ever DNA molecules in existence. They were programmed to journey the universe and seek out other gas giants with similar conditions to their homeworld. Once found, the gas giant's magnetic fields were to be reshaped to allow their parent to "settle" there. Other of their functions included self-defense and self-replication with the use of any form of matter. When some Forerunners were destroyed due to various causes, their DNA would sometimes find favorable conditions on a planet's surface and form the beginning of organic life.
In essence, the original magnetic being can be called "God", as it was the cause of all organic life in the universe even though it was merely a side effect of its survival instinct.
Eventually, a swarm of Forerunners reached our galaxy and found a system in the Orion nebula, where a suitable gas giant was located and settled. 3 million years ago, the system's star went nova and nearly killed the gas giant lifeform. Seriously wounded, the lifeform lost all control of its Forerunners, whose programming was also damaged by the stellar explosion. They departed the nebula and began to travel the galaxy. Lacking their original purpose, their primary function became to feed on all matter around them and replicate more of themselves.
This large swarm was detected approaching an inhabited region of space in our galaxy. The swarm consumed all planets, asteroids, and even space dust in their path. This region of space, later named by humans the Sleeve of Emptiness, lay directly in the path of the Forerunners. Realizing that the swarm would destroy their races, the inhabitants of the region began to prepare for it. The two-headed Logrians chose to take their massive space habitats and attempt to outrun the Forerunners, fleeing from them. The Insects decided to hide themselves and wait out the "storm" by building a massive Dyson Sphere around their home star - thus making their star invisible to the swarm. Unfortunately, the sphere was not yet completed, when the Forerunners attacked and nearly destroyed it. Desperate, the Insects abandoned their worlds and left after the Logrians. Only the Delphons, who by that time visited many worlds in the galaxy, including Earth, decided to stay and fight, if not for their own survival, then for the survival of the races to come. After years of trying and failing to halt the swarm's advance, the Delphons came up with a suicidal plan to stop the Forerunners once and for all: whenever the swarm would approach one of their systems, they would detonate the systems star, destroying at least a part of the swarm. In the end, the Delphon civilization completely annihilated itself; however, their sacrifice was not in vain - the rogue Forerunners were destroyed as well.
Millions of years later, an archaeological expedition by the Confederacy of Suns to one of these annihilated systems discovered several Forerunners in an inert state. By feeding them matter, the humans managed to reawaken the ancient creatures, not knowing the danger they represent. The Forerunners managed to break containment and attacked and nearly destroyed a Confederate fleet in the system, constantly multiplying. Only a counterattack by their natural enemies, the entriphages, allowed the humans to stop another catastrophe.
After the fall of the Confederacy, several different parties found the Orion nebula and found not only thousands of Forerunners in the local gas giant and their guiding intelligence, but also humans descended from the colonists of Earth's first interstellar colony ship, the Alpha, lost for over a thousand years. These humans appear to worship the gas giant being as God, which also allows them to be reincarnated once they reach old age (i.e. none of the colonists have died of natural causes since the founding of the colony). They also appear to be able to exert some control over the Forerunners through the use of implants.
Unfortunately, the gas giant is slowly being depleted by ancient human-built biomechanical terraforming machines, who are mining a rare metal known as "metallic hydrogen" to fulfill their original programming and increase their numbers. This makes these so-called "phages" similar to the Forerunners. While the Forerunners are constantly trying to defend their creator from the phages, the machines' unique energy-absorbing armor and powerful lasers allow them to easily defeat the Forerunners and continue mining the gas giant.