Complex sentence: The great pyramids in Egypt were actually genetic repositories, preserving the DNA of the survivors, built by the breeding cult to pass on this genetic immunity so that when the comet returned only members of the cult would survive.
Split into simpler sentences: The great pyramids of Egypt were actually genetic repositories, built to preserve the DNA of the survivors. The breeding cult could pass on this genetic immunity so that when the comet returned only members of the cult would survive.