Complex sentence: By summoning swarms of Exolon, he can manifest what appears to be darkness; because the Exolon feed on souls, swarms of it appear--at least to the Phalanx--as the exposed soul of a living being.
Split into simpler sentences: By summoning swarms of Exolon, he can manifest what appears to be darkness. Because the Exolon feed on souls, swarms of it appear--at least to the Phalanx--as the exposed soul of a living being.