Age of Abandonment
It is speculated that the Age of Abandonment was the period of time when the Forerunners activated the Halo Array, or, as the Covenant put it, "embarked on the Great Journey". The Covenant believes that the Forerunners have became gods, but in fact they disappeared from the universe because they activated the Halo Array in order to contain the recent Flood infection, thereby killing themselves and all other sentient life in the galaxy. This event happened about 100,000 years prior to 2552.
Progression of the Ages
Leader, or Chieftain: Abandonment.
Congregation, or Pack: The First Age! Ignorance and fear!