rule with an iron fist
To rule, govern, or control a group or population with complete, typically tyrannical authority over all aspects of life, work, etc. A noun or pronoun can be used between "rule" and "with." He rules with an iron fist, and moves swiftly to gain control over any entity that is not already in his grasp. She has ruled this company with an iron fist for three decades, and it's going to be difficult for her to let go of control.
rule with an iron fist
Fig. to rule in a very stern manner. The dictator ruled with an iron fist and terrified the citizens. My boss rules with an iron fist. I'm looking for a new job.