Complex sentence: At the time, Cao Cao had yet to designate one of his sons as his heir apparent and he was considering Cao Zhi, so he secretly sought the opinions of his subjects, including Cui Yan.
Split into simpler sentences: At the time, Cao Cao had yet to designate one of his sons as his heir apparent and he was considering Cao Zhi. He secretly sought the opinions of his subjects, including Cui Yan.