The Byavoor are an alien species native to the Calyx Sector.
It is believed that the Yu'vath limited the Byavoors thought processes making them more docile.
A former slave race of the Yu'vath that were used incorporated into their master's dark rituals to feed their hunger. Due to the docility of the Byavoor, it spared the entire race from extinction by the Imperium as they were deemed not a threat during the Angevin Crusade.
In current times the Byavoor are scattered around the Calixis Sector acting as mercenaries, and are described as little more than "sentient cattle".
- Rogue Trader: The Koronus Bestiary (2012)