Complex sentence: Dante placed Ripheus in the sixth sphere of Jupiter (Paradiso Canto XX: 1-72), the realm of those who personified justice (Cantos 18 through 20).
Split into simpler sentences: Dante placed Ripheus in the sixth sphere of Jupiter (Paradiso Canto XX: 1-72). Sphere 6 (Jupiter) was for those who personified justice (Cantos 18 through 20).