on pain of

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

* * *

on pain of

Under liability to the penalty of

● pain

* * *

on (or under) pain of

the penalty for disobedience or shortcoming being

all persons are commanded to keep silent on pain of imprisonment