Second Cleax Invasion
The Second Cleax Invasion began in 3202 B.E. and was precipitated by the destruction of the Sheol Locus, which had been built by the Vyar after the First Cleax Invasion to keep the Cleax out of Starmourn, though the Younger Races were unaware of this, and a Krona expeditionary force to the Permian Abyss damaged the Sheol Locus in 3241 B.E. with gravity munitions. As the Y'saari would later explain after having discovered what the impetuous Younger Races had done, the Locus had been built in the Permian Abyss because its technology required an area as free from gravitational fields as possible - thus why the gravity munitions were able to damage it.
39 years after the Locus's signal was changed, in 3202 B.E., the Cleax invaded, arriving through the Voidgates in incredible numbers to destroy the Locus and ensure its whispers of death and pain never plagued their minds again. They then swarmed from there towards central Starmourn, spreading from Voidgate to Voidgate like a virus, slaughtering all whom they encountered and taking whatever was useful to them.
Though we know now that the Cleax are a race with two distinct forms. During the entire Second Invasion, they saw only one of the forms - a disgusting - by humanoid standards - creature with 16 stubby legs, a long slug-like body covered with nodules that that can emit toxic gas, multiple manipulator appendages, a terrifying-looking maw, and long, flowing antenna.
These creatures piloted strange-looking starships apparently of their own manufacture, and the individual Cleax, as the Younger Races learned, are capable of surviving hard vacuum with nothing but breathing apparatus. On closing with the enemy, they would often use small personal 'jetpacks' attached to their bodies to propel themselves off their ships, attach themselves to the exterior of enemy starships, and then use their version of neutron welders to penetrate the ship's skin, enter it, kill or enslave those aboard, and take the ship for their own to add to their fleet.
The Cleax were eventually stopped by an alliance of over 15 Younger Races inhabiting central and western Starmourn, with assistance from the Y'saari, and a final, massive battle taking place just corewards of the Permian Abyss, where the Breakers are now.
In the aftermath, the Y'saari moved the Vyar's Aegispheres in to protect and control the central Voidgates of Starmourn, and imposed the Covenant Mark as necessary currency to use them, essentially establishing the Mark as the dominant and eventually only currency of Starmourn.