preposition
or under pain of also upon pain of
: under penalty or punishment of — used with a following specifying word or phrase to indicate that the thing specified is invoked as a threat that will be fulfilled
was told not to leave the country on pain of death
forbidden to pronounce his or her own name, on pain of incurring some great evil — J.G.Frazer
bringing pressure upon the commissioner … to appoint to office his favorites under pain of meeting his opposition in Congress — R.M.Lovett
or archaically or obsoletely that the thing specified is something that will be forfeited
that every one upon pain of life should return to their houses — James Howell
or is a crime or offense with which one will be charged
that every one should open his shop under pain of rebellion — James Howell