Possible Clues:

• Unspecified, but invariably unpleasant, alternative
• Unfinished threat
• Threatening option (2 words)
• Threat conclusion
• If not, then - sounds like a threat
• ''Otherwise . . .''
• If not, then - vaguely threatening
• '___ what?' (response to a threat)
• '...unless you want trouble'
• Ultimate words in an ultimatum