In few words, the Lightbringers are a death cult.
The order formed among a sect of humans that were one of the last surviving groups of the race. Distraught and fearing the heat death of the universe; religious leaders, political heads, and most positions of power dissolved into a belief that life was supposed to end in order to become anew. Those believing in the Lightbringers teachings turned toward the desire for an apocalypse. They awaited a new 'big bang' after the current one fully fizzled out, believing that their souls would be reborn within the new universe or of the desire for it not to be a true end.
It became an overwhelmingly common belief as the stars slowly went out one by one. Once swimming in a void, scientists were quickly overwhelmed by those who had latched onto the belief and soon had no choice but to be indoctrinated into it as well.
Several generations have passed with the Lightbringers carefully culling their numbers and replenishing them with the only goal of ensuring the end of all life within the universe. Most of which has come by their own hand as the stars slowly went out. The Lightbringers have divided their numbers and sailed across the stars to succeed in this task they have come to believe in.
The Lightbringers are very careful with their population with most acquired acolytes joining the frontlines to kill and be killed for the cause. Internal structures of the order would ensure their population was always at a certain number in their systems so that the culling of life would never stop for their ancestors souls' sakes.
When at a certain number however, repopulation would be treated as a ritual rite where one parent must sacrifice themselves and pour their blood over the newborn infant in an act meant to balance the life they had created against their beliefs in order to continue to fulfill those same beliefs.
The Lightbringers often take a non-hostile approach in new systems, taking time to ally with current occupants and establish themselves. Once able to gain a substantial postion in the system and adequate resources, they quickly turn to a convert or condemn method politically, then to outright genocide.
Defectors and outsiders of the cult often nickname the Lightbringers, 'the Purgers' after the cult decided to launch the Great Purge. This removed their desire to convert and progressed to only killing newly acquired population as their armies were slowly replaced with created life, clones, that they considered 'without soul' and therefore not hindering to their goal of an empty void ready to welcome the next 'big bang'.