A phrase indicating that failure to comply would result in a penalty.
“On pain of” was a legal phrase meaning that a person was required to do something under threat of a penalty if they failed to comply.
Manorial court records frequently used this wording when ordering tenants to carry out duties, repair property, maintain boundaries or obey local customs.
The phrase reflects the authority of the court to enforce obligations through fines or other penalties.
See also:
Amercement
Presentment
Custom
Court Baron