The Severance
On June 12th, 2290 - sixty years after the second colony ship arrived at the Celassi star system and entered orbit around Raana - the Celassi quantum gate mysteriously shut down, making the return trip to Alpha Centauri and Earth impossible. This happened almost 200 years ago, and triggered the beginning of a period called The Great Chaos (2290-2315) - which led into Ikaanos' "Dark Age" of 2315-2440, which only came to an end less than five decades ago thanks to the stabilization wrought by the current Chancellor Mordecai Adler.
The Historical Society continues the debate why The Severance happened. It remains the biggest mystery currently on the minds of the humans of Raana. A worrisome sound recording logged just prior to the malfunction hints that something attacked the quantum gate at Alpha Centauri. The crucial question is by who? Or by what? No other intelligent alien civilizations are known to humanity - other than the mysterious builders of the quantum gates that link dozen of star systems together - and there were no brooding conflicts between the central star systems when the severance occurred.