On pain of
The phrase on pain of death was once common in law to express prohibitions the violation of which would result in punishment by execution. The phrase has passed into lay contexts, in which it is used facetiously. But it remains as a shortened phrase, “on pain of,” in legal usage. E.g., Is it reasonable to require prison employees to have foreseen, on pain of section 1983 damage liability, the future of prisoners’ rights?