an iron hand
1. A particularly severe, forceful, and uncompromising manner of doing something. Tom is in for it now with his wife. She might seem like a nice lady to us, but she runs her house with an iron hand.
2. A person who does things in such a manner. The new leader of the country rose to power with promises of democracy and equality, but he soon proved to be an iron hand as his despotic intentions came to light.
iron hand
A manner that is severe, forceful, or uncompromising. Grandma runs the house with an iron hand—if you break a rule, you're grounded for weeks.