on pain of meaning

• [American slang]
  prep., formal At the risk of; under penalty of. The workers went on strike on pain of losing their jobs. The rebels had to swear under pain of death never to confess where their comrades were hiding.
  

• pain:    Noun: pain &n ...
• a pain in the ass:    [American slang]Fi ...
• a pain in the neck:    [American slang]Fi ...

Examples

1. it was prohibited on pain of death.
2. if you spring me from this cell, i swear on pain of death
3. i swear on pain of death that i will take your love to you
4. if you spring me from this cell, i swear on pain of death
5. and never return on pain of death