Complex sentence: Ultimately, Prophet Five's endgame remains a mystery, as the Twelve were slaughtered by Kelly Peyton on Sloane's orders, though since Irina and Sloane were apparently after immortality, it can be assumed the Twelve were after the secret as well.
Split into simpler sentences: Ultimately, Prophet Five's endgame remains a mystery, as the Twelve were slaughtered by Kelly Peyton on Sloane's orders. Since Irina and Sloane were apparently after immortality, it may be assumed the Twelve were after the secret as well.