On/under pain of something
Definition 1 of 1
Pro Tip 1/3Severity Highlight
Indicates a severe or serious consequence for not complying with a demand or rule. 
Access to this database is restricted, on pain of severe disciplinary action.
Pro Tip 2/3Legal Usage
Often used in legal or formal contexts to stress compulsory obedience to laws or rules. 
The signed contract must be honored, under pain of legal fines.
Pro Tip 3/3Cultural Weight
The phrase can carry historical gravity, especially when used in contexts relating to past authoritarian practices. 
The medieval decree was to be followed, under pain of death.