This video attempts to answer the question: What links narcissistic personality disorder (NPD) with borderline personality disorder (BPD)?
What the question is really asking here is: Is there some sort of characteristic that in some meaningful way, really links these two together conceptually, so that we can look at the same origin or central characteristic in both disorders.
With NPD, we see grandiosity, having fantasies of success and power, jealousy, entitlement, arrogance, feeling special or unique, manipulating other people, needing admiration, and having a lack of empathy. Borderline personality disorder has nine symptoms: frantic efforts to avoid abandonment, splitting in relationships (unstable relationship pattern), identity disturbance (unstable self-image), impulsivity, suicidal behavior, emotional dysregulation, a feeling of emptiness anger, and paranoia.
Ronningstam, E., & Baskin-Sommers, A. R. (2013). Fear and decision-making in narcissistic personality disorder-a link between psychoanalysis and neuroscience. Dialogues in clinical neuroscience, 15(2), 191-201.