The eighth monastery is a secret society of ageless higher humans, fighting the determinism war, sworn to destroy the Jeém and their seven monasteries because of what they percieve as the deliberate destruction of the human race. The eighth are the remains of the people from the old times, the leftovers of civilisation from before the fall, they seek to free feral humans (who are often culled) from the tyranny of the Illúme because they believe that the Jeém engineered the current status quo.
Many members of the eighth are ex-soldiers from the war and they alone, along with the Jeém remember from before the collapse. However they disagree that they would be dead if it wasn't for the Jeém. During the war these individuals often bought their agelessness at the expense of others: agreeing to fight for the have's using the dymorphic phage as payment.
The Eighth blame the Jeém for the fall, and secretly aligned themselves as a group of higher humans refusing to bow to the call to become rendered Illúme: willing avenge themselves against the thinking machines that they think would have destroyed them all.
The Eighth are often at work and have even infiltrated certain ranks of the Illúme they far our gun the monasteries in technology and only the brutalus can counter them in anything like equal terms.