In the New Sith Empire, a legacy was the offspring of a Sith Lord (or, more rarely, Lady). Though Sith could and occasionally did bear children with willing partners, most legacies were sired on breeding slaves who were kept for that purpose and selected for fertility and genetic fitness. The minders who oversaw the breeding facilities endeavored to match Sith Lords with identical species rather than risk increased hybridization, though interbreeding among Humans and various Near-Human species was tolerated to a degree.
Healthy offspring who were not Force-sensitive were funneled into other training programs, such as for the military or Sith Intelligence; unhealthy offspring were euthanized. Forceful offspring were sent to the various Sith academies for training. Legacies knew what they were, but rarely knew who their parents were.