"The Advent were originally a pseudo-religious sect of humans who sought spiritual release and escape on the frontier of human space during the great interstellar wars that preceded the rise of the Trade Order. Over time, their culture evolved a socially liberal, highly laissez-faire attitude towards the use of biological enhancers such as synthetic neurochemicals and "cerebral integration technologies" as means toward self-actualisation, resulting in a burgeoning collective mentality calling itself the "Unity". When their society was discovered by the market analysis teams of the nascent Trade Order, their collectivist practices were branded as perverse deviations, and the sect was exiled from Trader space. Over centuries of exile in far-flung regions of the galaxy, the sect continued to plumb the depths of biological mutability and posthumanity, developing powerful "PsiTech" tools, which enabled through unknown means a measure of psychic manipulation over local physical phenomena, as well as other technologies far surpassing anything the Trade Order had conceived of. A millennium after their betrayal, they have returned as the "Advent" to visit vengeance on the Traders and bring humanity together under the peace of the Unity.
The Advent favor cheap, lightly-armored ships, preferring the elegance and sophistication of advanced force field systems over clumsy metal armour. Their weapons are almost exclusively based upon directed energy; lasers and plasma weapons are prominent on every warship, and certain units have PsiTech abilities which allow the crew or some uniquely modified members of the crew to exert limited control over local phenomena, for instance in manipulating the crews of other ships or planets. Their units are very sleek and aesthetic, featuring artistically-minded spirals and organic shapes, unlike the more utilitarian designs favored by the TEC. The Advent prefer Desert planets such as the one they were exiled from, and can research population bonuses for those worlds. Being evangelistic, they can research culture-spreading buildings, in this case Temples of Communion, before either of the other two races, and can spread culture more effectively than the others. Advent fleets, being cheap, tend to maintain large numbers of craft, but rather than treating their units as expendable, Advent prefer to use a variety of highly specialised vessels which function well in conjunctive use, and many Advent ships are designed to improve the performance of nearby allies, such as by improving shield strength, reducing weapon cooldown times, or stealing antimatter from enemy ships and distributing it to allies to power their own special abilities."
That is pretty much it. Everything else is assumed for fanon.