Complex sentence: Thereafter the Heliumites and Tharks defeat the First Born, and Issus herself is killed; but Dejah Thoris, Thuvia, and Phaidor are imprisoned in the Temple of the Sun, of whose rooms each opens only once per year.
Split into simpler sentences: Thereafter the Heliumites and Tharks defeat the First Born, and Issus herself is killed. But Dejah Thoris, Thuvia, and Phaidor are imprisoned in the Temple of the Sun, of whose rooms each opens only once per year.