The Diaspora, also called the First Diaspora or the Hegira, was the exodus of several different groups and individuals from the Homeworld during and immediately following the anchoring of the thread. The single largest group was the Eremites; other groups included several Great Houses, such as the ruling House Ixion.
Most groups that left during the Diaspora were motivated by their fear that the radically-transformed Homeworld had devolved into madness or would soon fall. Others, like the Eremites, left for political reasons, rejecting the new order of "Urizen" and the technologies that followed. Eremite mythology depicted the Diaspora as the "Hegira", a circular pilgrimage that would end in their return to prominence among the Houses after the "Redux", when they would defend and scourge the Homeworld at its hour of greatest need.
During the War in Heaven, Academician Devonire convinced many Great Houses that had left during the Diaspora to return to the Homeworld. The Diaspora was frequently called the "First Diaspora" in anticipation of a Second Diaspora from the Homeworld. (PROSE: The Book of the War)