The Battle of the Greater Ark was one of the final battles of the Forerunner-Flood War and saw the decimation of almost all remaining Forerunner forces just prior to the firing of the Halo Array during the Great Purification in 97,445 BCE. Having lost nearly every inhabited system in the galaxy, the remaining Forerunner military forces including a single senescent halo, Omega Halo, retreated to the Greater Ark to have a last stand against the overwhelming Flood forces. It was there that the Librarian, IsoDidact, Ur-Didact, and the Master Builder met for the final time to try and do whatever they could to stop the Flood from completely wiping out all remaining Forerunners.
At the onset of the battle, Omega Halo was used to try and stop the Flood forces, but they implemented the ancient Precursor star roads in order to quicken their attack and avoid much of the halo’s pulse. Offensive Bias, the Forerunner ancilla tasked with command of the remaining Forerunner military, fought valiantly against Mendicant Bias and the Flood forces, but were quickly overrun and the Greater Ark was destroyed. Though the IsoDidact, Librarian, and Ur-Didact all managed to escape, nearly every other remaining Forerunner was killed, presumably including the Master Builder.