Complex sentence: Their leader, Bellal, explains that the city was built by the Exxilons' ancestors, who were once capable of space travel, and built the city to be capable of maintaining, repairing, and protecting itself.
Split into simpler sentences: Their leader, Bellal, explains that the city was built by the Exxilons' ancestors, who were once capable of space travel. The ancient Exxilons built the city to be capable of maintaining, repairing, and protecting itself.