The Burn are a hostile race in the VayneLine universe and one of the few races that was not subsumed within the Aelisha race/protectorate. The Burn at their core are a gaseous, telepathic race / consciousness much different from many of the material races. With the advent of cybernetics and mechanical bodies the Burn were able to leave their gas giant planet and being conquering. Many know them only now as a mechanical race that is disturbing in their pursuit of efficiency.
For many intents the Burn can be viewed as a mechanical or cyber race, but it is not completely true given their origins. However, no one is going to argue the semantics as their fleets destroy colony after colony. Their armadas are potent yet easily replaceable and a rival to Aelisha power, even if the Aelisha do not admit it.
There are many questions why the Burn would attack the Solarians when it will trigger Aelisha hostilities when they had lived ‘in peace’ or at least left alone for so long.