King Zhaoxiang of Qin
Because King Zhaoxiang had not yet legally come of age (traditionally at the age of 20), his mother, who was now known as Queen Dowager Xuan, became the regent. She was supported by her brothers, Wei Ran and Mi Rong (羋戎), as well as two other sons, Prince Yi (公子悝) and Prince Fu (公子巿), the four of them collectively known as the "Four Nobles" (四貴).
Li Mu
Getting rid of Li Mu became a necessity for Qin to conquer Zhao and unify China. Qin sent spies to the Zhao court, bribing key courtiers in order to persuade the King of Zhao that Li Mu was planning to revolt. The plan succeeded - Li Mu was arrested and soon executed on the king's orders. According to Sima Qian (chapter 102) this happened when King Qian, 'whose mother was a dancing girl', came to the throne.