The Children of the Night were the dominant species of the Milky Way galaxy for twenty million years before they were wiped out and replaced by the Xul.
The Children of the Night went extinct several million years before humans evolved, so they are never seen. Their only known anatomical feature is their strong aversion to light.
The Children of the Night were also "psychovores," feeding on the minds of other species through unknown means. They often enslaved entire planets for this purpose.
The Children of the Night rose to galactic dominance 30 million years ago, following the disappearance of the One Mind that had previously ruled the galaxy. The Children exterminated every species that could threaten them and enslaved others for feeding purposes. Their rule of the galaxy lasted for twenty million years before the Xul arose to challenge them. Over the course of two million years, the Xul exterminated the Children and their entire civilization, presumably killing off their slaves as well. Afterward, the Xul assumed dominance of the galaxy.