Complex sentence: Morisaki explains that long ago when humankind was young, it needed the guidance of Quetzalcoatls (keepers of the dead) until humans matured and no longer needed them, so they went underground along with a few humans who joined them.
Split into simpler sentences: Morisaki explains that long ago when humankind was young, it needed the guidance of Quetzalcoatls, keepers of the dead, until humans matured and no longer needed them. They went underground along with a few humans who joined them.