They will bad mouth you to everyone but it doesn't matter because people know what they are like. 

Not all narcissistic people are violent, but I've met some who are.

They have revenge tendencies. Your property or communal standing can be messed with. I've never seen them take on people physically though: they don't want their pretty faces and clothing messed up. (Plus it has to be on their time).

Rage, possibly murder, if the narcissist also has an underlying psychological disorder

They become degrading, and can also become violent.

My personal narcissist (my stepdad) ignores us when I make him angry. We are going on 9 months of ex-communication because I gave a response he didn't like to a question he asked. I spoke the truth - he tires easily of the truth if it doesn't fit his version of reality. 

I used to bow down, beg to be forgiven, and repeat the lies that he prefers to keep my mom happy, but after 30+ years, I am finally enforcing my borders and I am not seeking forgiveness. I kind of miss my mom, but she chose him over me and my kids many years ago and I have just grown weary of the fight. She is 80 now and I fear I may not see her again because of him.