Complex sentence: The great pyramids in Egypt were actually genetic repositories, preserving the DNA of the survivors, built by an ancient blood cult that passed on this genetic immunity so that when the comet returned only members of the cult would survive.
Split into simpler sentences: The great pyramids in Egypt were actually genetic repositories, preserving the DNA of the survivors, built by an ancient blood cult that passed on this genetic immunity to selected members to keep this antibody against the return of the comet. When the comet returned only members of the cult would survive.