Skaald were a race of spacefaring reptilian humanoids originally from the planet Skaala in Eta Prometheus System, Prometheus Sector. They were wiped out in the course of the Human-Skaald War.
Biology
The nearest Terran analog to the Skaald would be the chameleon - the Skaald race evolved from a Skaalan cliff-dwelling ambush predator that would prey on a Skaalan avian living among the rocks, specializing in particular in blending into the rocks until the avian came close to its nest, then killing it with a fast pounce and sharp bite. As it rose to intelligence, the Skaald's depth perception and innate grasp of trajectory lent well to ballistics, flight and eventually space exploration. They had a robust constitution, able to handle broad impacts in general and falling from great heights in particular with relative nonchalance. Although they didn't evolve in zero-gravity, the racial penchant for cliffs and the ability to hang from any surface allowed them to adapt to space.
Culture
From what little is known about Skaald culture, their perch atop the Skaalan food chain lent to them a genetic arrogance - unlike humans, who remained tasty targets even after developing sentience, there was little to dissuade the Skaald from believing that they were the pinnacle of creation and coming up with creation myths and religion to reflect that. The culture revolved around a quasi-clannish structure that persisted - familial bonds first and foremost - up through the days of its internal strife, which were not quite as sensitive as humans as to balk at slaughtering entire clans.
History
see main article at Human-Skaald War
What little humanity knows about Skaald history comes from bits and pieces gleaned from Skaald installations - a war of planet-killers tends to leave no records, and so the history of Skaala is almost unknown. What humanity is aware of, however, was the way in which a Skaald cruiser jumped into the space orbiting a Prometheus Sector colony and started bombarding the surface. The collection of Van Ryan Nebula colonies that would become the Spartan Brigade fought back, pounding out ships and sending out men, and fought the invaders to a standstill. It became a brutal war of attrition, human ships sacrificing themselves to fight off Skaald battlegroups, conscripts forming a sea of bodies to hold back invaders on the surface. The horror stories from Prometheus Sector would forever mold the consciousness of the Spartan people, devaluing the self to loft an ideal onto a pedestal, preservation of the state above all.
The League's total involvement in the war, sending troops and ships in to fight the Skaald invaders, triggered a change in tactics. No more did the Skaald strike to claim planets. Now it lobbed wave after wave of virus warheads and tectonic disruptors at League planets after smashing through guardian fleets. In horror and anger, Spartan fleets replied in kind - shattering Skaald battlegroups, then glassing entire planets, with atomic warheads. This shift of tactics slowly caught on with the League Navy at large, and there were many more League fleets than Skaald battlegroups.
Finally the battle came back to Skaala, the last Skaald stronghold, and because repeated attempts could not reach the planet through the impenetrable thicket of Skaald ships, a new strategy was required. Scientist Tag Behrwell and his experimental field generator based on a Skaalan planetary core quencher. The end result was devastation - the star Eta Prometheus was snuffed, and the gravitational flux that resulted ripped everything that remained in-system apart. Humanity had met intelligent life for the first time, judged itself incapable of coexisting with it, and destroyed it.