Complex sentence: However, he could not be the next emperor after the Emperor died, because Empress-consort Uno-no-Sarara had so much power that she declared that her son, Prince Kusakabe, should be appointed the next emperor.
Split into simpler sentences: However, he could not be the next emperor after the Emperor died. Because Empress-consort Uno-no-Sarara had so much power that she declared that her son, Prince Kusakabe, should be appointed the next emperor.