Narcissism is extremely destructive for any one that comes in touch with it especially. If you are in a relationship with a narcissist this is particularly acute. Narcissists exhibit the follow traits.
Grandiose sense of importance. - They think they are more important than most other people.
Preoccupation with unlimited success.
Belief that the person is special and unique.
Exploitative of others. - Possibly the most important and most harmful trait
Lack of empathy. - Just a like a Psychopath they won't care about others feelings.
Arrogance.
Jealousy of others.
In a relationship these factors make the non-narcissistic partner feel worthless often through the use of gas lighting (the method of exploiting someone by making them feel like their own memories of a situation are false and they are crazy). They also tend to try and control peoples behaviours and have an absolute sense of that their own opinion is right.
Unfortunately there is no cure for narcissism and the only way to deal with a situation where you have recognised these traits is to get away from.