Narcissistic abusers and otherwise toxic people feel very threatened when their excessive sense of entitlement, false sense of superiority and grandiose sense of self are challenged in any way. They are prone to making unreasonable demands on others – while punishing you for not living up to their impossible to reach expectations. Rather than tackle disagreements or compromises maturely, they set out to divert you from your right to have your own identity and perspective by attempting to instill fear in you about the consequences of disagreeing or complying with their demands. To them, any challenge results in an ultimatum and "do this or I'll do that" becomes their daily mantra. If someone's reaction to you setting boundaries or having a differing opinion from your own is to threaten you into submission, whether it's a thinly veiled threat or an overt admission of what they plan to do, this is a red flag of someone who has a high degree of entitlement and has no plans of compromising. Take threats seriously and show the narcissist you mean business; document threats and report them whenever possible and legally feasible.
