Glacius was a protoss research world.
Prior and/or during the Great War, Khalai scientists planned on reviving the purifier program. It was there that Praetor Fenix replicated his personality and memories into a prototype warrior made in his image.
During the End War, the Tal'darim, under command of first ascendant Alarak, invaded Glacius in an attempt to kill the Khalai scientists and destroy the purifier technology vault. Artanis had the Khalai scientists evacuated, and sent a force to retrieve the weaponry before the Tal'darim could destroy it.