Complex sentence: The Mentors were also seen to be selling weapons to their less advanced neighbours from the planet Thoros Alpha and were holding the Alphan King, King Yrcanos as a hostage.
Split into simpler sentences: The Mentors were also seen to be selling weapons to the less advanced neighbours from the planet Thoros Alpha. They had captured the Alphan King, King Yrcanos, and were holding him as a hostage.