General:
* Terror - This is the most straightforward option, but it focuses on the emotional state rather than the action.
* Intimidation - This emphasizes the act of making someone afraid.
* Harassment - This suggests repeated and persistent attempts to frighten or distress.
Specific:
* Reign of terror - This refers to a period of widespread fear and violence.
* Tyrant - This is the noun for someone who terrorizes others.
* Terrorism - This refers to the use of violence or threats to intimidate a population or government.
Ultimately, the best noun depends on the specific situation and the nuance you want to convey.