Complex sentence: Earlier theories have favoured the idea that Egypt was split in two realms during Peribsen's time or that he was a heretic, who sought to start a new monotheistic religion with Seth as the only worshipped god.
Split into simpler sentences: Earlier theories have favoured the idea that Egypt was split in two realms during Peribsen's time. Furthermore it was proposed that Peribsen was an heretic, who sought to start a new monotheistic religion with Seth as the only worshipped god.