Complex sentence: He allies himself with Ricimer, "de facto" ruler of Rome, and marries his daughter Alypia to him, to strengthen the relationship and end the hostilities between the Eastern and Western Empire.
Split into simpler sentences: He allies himself with Ricimer, "de facto" ruler of Rome, and marries his daughter Alypia to him. This to strengthen the relationship and end the hostilities between the Eastern and Western Empire.