Forerunner (Halo)

The forerunners are the ones who created the seven Halos all over the milky way galaxy, in order to study and contain the flood, and in case of an outbrake, activate Halo´s defences and kill all sentient life in the galaxy to starve them to dead. They´re beleved to be extincted because they were forced to activate Halo to stop an outbreak, of course, it is unknow wether if they died on halo´s activation or not.

Theories and History
There are strong rumors that installation 00, "the Ark", is buried under new Mombasa, this may led to the connection between Humans and Forerunners and what the prophets and Tartarus know something Humans and the other covenant don´t:

It is belived that Humans are evolved forerunners, or their selected species to "reclaim" their tech. That will explain why the ark is on Earth. The prophets and the elites found the forerunner tech, and started forming the covenant, as a religious society worshiping forerunners. Later, somehow, the prophets find out about humanity and their right to the forerunner tech, so they started the war with humanity.

Sources:
 * Fan Theories
 * The aditional booklet with Halo 2 collector´s edition, where it is described the conversation between two elites, in wich one of them asks the other why instead of fight the humans, the Convenant invite us to join them. This means that the prophets know something the reste of the covies don´t.
 * 343 Guilty Spark, refers to Humans as "Reclaimers", and Humans are the only ones who can insert the index in the Control Center.
 * At the final level of Halo 2, Tartarus forces Miranda to insert the index, why he couldnt´t do it by himself????