This is semi-straight from the book and I've twisted some facts to better suite the lore created....
The Advent, which the society is known collectively as The Unity, are a highly advanced civilization that relies heavily on spiritual, psychic, and cybernetic technolgies. A sense of elitism permeates the Unity, and is most strongly expressed with the high value they place on psionic abilities.
Silent Ones; those who are unable to participate in the Unity's group mind, are exceptionally rare and universally shunned.
At an early age, Advent children are taught to seek enlightenment in whichever way naturally suits them and it is common to see even the very young using neurochemicals or artificial implants to augment their mental abilities. This advanced level of integration with their technology - known as PsiTech - is largely responsible for the Advent's strange sense of collectivism and allows them to share their thoughts and feelings on an almost intimate level.
Navigating, interpreting and shaping this sea of ideas are highly prized skills in Advent culture and one's mastery in these are the basis of the unity's social hierarchy. Females in particular demonstrate a natural prowess and have come to dominate the higher castes, called Coalescences.
When the Trade Order forced the Advent's ancestors off their home world, it scarred their psyche leaving a strong desire for retribution, which through time was turned into a prophecy, later rebirthed by the Empress. This desire though blinded through countless years of peace was the drive that The Advent possessed contributing to their technological advancements that surpassed the TEC by eons. By the end of the 1000 years of solitude and the birth of the Empire, the Advent went to fulfill their long lost prophecy.
The unity undivided in their goals, are divided in emphasis, those of Harmony and those of Hostility...
The followers of Harmony places emphasis on the collective mind and of Advent cultural.
The followers of Hostility, a sect that was born during the birth of the empire from the new council under the empress's lieutenants, placing an emphasis on war / retribution and technology mostly in PsiTech.
Both follow the will of the unity in ways that make the Advents empire undivided and unflinching.