Complex sentence: Tissaphernes, who found out about Cyrus the Younger's plan to assassinate his brother, informed the king about the conspiracy, who then had Cyrus imprisoned, but by the intercession of his mother Parysatis, Cyrus was pardoned and sent back to his satrapy.
Split into simpler sentences: Tissaphernes, who found out about Cyrus the Younger's plan to assassinate his brother, informed the king about the conspiracy, who then had Cyrus imprisoned. But by the intercession of his mother Parysatis, Cyrus was pardoned and sent back to his satrapy.