The Great Galactic War.

The Republic enjoyed a thousand years of peace after the The Great Hyperlane Invasion by the sinn.  The old sinn emperor, Angard Shadow, died, leaving the throne to Kaga Diin.  Diin was then challenged by the irascible, illegitimate, son of Shadow, Draco Ravorn.  Draco fought and killed Kaga Diin in a duel right outside the latter's own palace, Draco stole the crown and initiated a new regime.  For too long had the Republic been at peace, and for too long had the sinn, the sworn enemies of the Republic, been reclusive and peaceful.  Draco built a massive fleet, bigger than the one used by Marak Shadow in the Great Hyperlane invasion.  Unlike the sinn emperors before him, who attacked lesser Republic worlds to slowly eat away at Republic power, Draco lead his fleet straight from Korduun in a spear-like lightening blitz to Eridu, the grand capital of the Republic.  The attack was both well timed and unexpected; the Republic had just gone through the Fourth Tuet Civil War; and no one had ever dared attack the capital of the Republic before.  Just as the troop transports had landed from the Tuet Cluster, the black, gothic ships of Draco appeared in Eridu's sky.  The great warships bombarded the city with photon missiles and laser cannons.  Draco's flagship, The Red Leviathan, smashed the spire of the Senate Palace onto the buildings below.  Sidon Troopers, Draco's elite soldiers, abducted several groups of senators and held them on their warships to ensure not being shot down.  The sinn blasted all planetary laser cannons, than the Central Eridu Barracks, killing all the clones within and destroying the squadron of ships there.  For three weeks the sinn bombarded the capital of the Republic, Sidon Troopers stormed the Senate and officially declared the end of the Galactic Republic.  The Supreme Consuls, Runan and Canscicio, fled with a battalion of Senate Guard into the lower levels of Eridu.  Draco wished to capture the rogue Consuls, so he announced he would kill a captive senator for every hour the Supreme Consuls stayed in hiding.  The Consuls were at a lose for what to do next.  That is, until Gasha warrior Trajan Larons and his apprentice, Taa Vas, snuck aboard the Red Leviathan and challenged Draco Ravorn to a duel.  Trajan was killed in the duel, but Taa was able to push Draco out of the ship; where he fell and was impaled on the Senate Palace.  Seeing their emperor dead, the sinn fled back to the Sinn Empire, but a Republic fleet from the Rylex Sector followed them to Korduun.  There the Republic fleet destroyed every last ship of the sinn fleet and radiated Korduun with a neutro bomb, making the world an uninhabitable, radiated wasteland for two-thousand years.  The descendants of Draco ruled an increasingly smaller and far weaker Sinn Empire from the planet Akshak.