Negotiator's outrageous demand, e.g.

• Negotiation roadblock
• Negotiations killer
• Fatal relationship problem
• Negotiations killer ... or, in a way, what each answer to a starred clue is
• Issue that halts negotiations
• Non-negotiable issue
• Accord halter
• Negotiator's outrageous demand, e.g

Wiktionary Word definitions in Wiktionary
n. (alternative spelling of deal breaker English)