Complex sentence: Their existence was kept hidden for millennia as they chosen a solitary life in their own community which prospered until the day the High Evolutionary activated his machines on an orbital space station, that switched off all mutant genes.
Split into simpler sentences: Their existence was kept hidden for millennia as they had chosen a solitary life in their own community. They prospered until the day the High Evolutionary activated his machines (from an orbital space station) that switched off all mutant genes.