Complex sentence: In the main event between Roman Reigns vs. AJ Styles in a Extreme Rules match for the WWE World Heavyweight Championship, where Reigns once again defeated Styles to retain his title, before Seth Rollins made his return from an injury and attacked Reigns.
Split into simpler sentences: In the main event between Roman Reigns vs. AJ Styles in a Extreme Rules match for the WWE World Heavyweight Championship, where Reigns once again defeated Styles to retain his title. After that main event match was over, Seth Rollins made his return from an injury and attacked Reigns.