The Cossus Incursia are a semi-insectile species that live in the interstitial space between dimensions.

Incursia bodies are grown - engineered - according to their purpose, and so they can take a wide variety of different forms. Generally, they are multi-legged, exoskeletal, and at least the size of a small car.

Where other species might augment themselves with metallic nanotechnology, the Incursia are designed to grow biomechanical devices as part of themselves, ranging from complex communications systems to heavy weapons.

Once, deep in the past, the Incursia were a relatively normal spacefaring species. However, by chance, in their home universe they were the only form of intelligent life to evolve. Combined with the particular psyche of those early Incursia, this led to an absolutely massive superiority complex - which in turn led to a substantial shock when they developed interdimensional travel and discovered that they were not only not alone, but also not even remotely significant, in the grand scheme of things.

The Incursia immediately decided that their security as a species, on an existential level, depended upon preventing anyone from coming to them. Aggressively, if necessary. They engineered their soldiers to live in the interstitial void and monitor it for signs of interdimensional travel. The plan was to spot any civilisation that developed large-scale hopping capabilities, and immediately annihilate them before they could intrude on the Incursia's home universe.

This strategy was mostly successful. Although the Incursia's watch was not omniscient, given the infinite number of possible universes, they were able to detect and destroy a number of civilisations whose interdimensional diaspora came too close to home.

Their first encounter with this universe, and their first stumbling point, was the Staiide. The Staiide had developed interdimensional travel relatively late, compared to their technological progress in other areas, and so their military potential came as a nasty surprise to the Incursia. Staiide defenders easily defeated the first Incursia attacks, allowing the Staiide interdimensional empire to spread in peace.

Over the millennia, however, the Staiide became complacent. They began to Upload themselves and fade away from the material worlds they had colonised. The Incursia crept in at the edges, slowly wearing the Staiide down until the remnants of the Empire posed no threat to them.

For hundreds of thousands of years, there was no danger to the Incursia from this universe. However, new civilisations were rising, and eventually they would once again develop the technology the Incursia feared. That time came at the end of the War of 4404. Small transits, individual Hoppers and small groups of nomad ships, had escaped the sentries' notice, but they could not fail to detect the use of the RAINDANCE superweapon at the Battle of Sargasso. The Incursia mobilised once more, and in 4409, the attack came.

After the War with the Galactic Defense League, the new Core Governance had hoped for a time of peace, but they were immediately forced to arms again, as Cossus Incursia fleets poured from their staging grounds on the northwest rim of the galaxy. At battles like New Kandahar, Governance forces fought desperate, drawn-out engagements against the swarming onslaught. Unlike the Staiide, the Governance did not have unusually advanced weapons available, and early losses were heavy.

Ultimately, though, Governance advances in AI were able to turn the tide. New Governance warships were simply too fast for the Incursia, and too capable of adapting and creating new weapons. Secretly, the Governance also benefited from the advice of surviving Staiide Combat Moths, who were able to provide information on Staiide technology and tactics which could be utilised against the Incursia. Eventually, the Incursia were driven back into the void.

The remaining Incursia sentries were forced to watch from the shadows of the interstitial void as the Governance developed its Expedition Ships and sent them out across the dimensions, fearful of the day they would stumble across the Incursia's home universe. So far, that day has not yet come.

Although the Incursia attack was effectively defeated by the Core Governance, it was logistically impossible to completely eradicate them from the universe. Some scattered survivors remained, lurking, hiding, in forgotten places. Now, in the Quiet Era, some of those vicious creatures, having nursed a hundred-thousand-year grudge, see their chance to crawl from their holes and take their revenge.

Reports of sightings of the old enemy are a worrying development, and one of the key missions of the knights of Cayd Rock is to eradicate Incursia creatures whenever they appear. Fortunately, these appearances are uncommon, for now.