Complex sentence: After fleeing the Covenant's destruction of the human world Reach, the human ship "Pillar of Autumn" made a random slipspace jump to avoid leading the Covenant to Earth and discovers a massive ringworld orbiting a gas giant.
Split into simpler sentences: After fleeing the Covenant's destruction of the human world Reach, the human ship "Pillar of Autumn" makes a random slipspace jump to avoid leading the Covenant to Earth. Arriving in uncharted space, the crew of the "Autumn" discover a massive ringworld orbiting a gas giant.