Relationships are entirely dependent on the personalities of the two individuals involved in it. While some are easy going and make it seem like a cakewalk, others have a tough time keeping the relationship stable and happy. If one of the partners is a narcissist, the relationship is going to suffer, as relationships are about mutual give-and-take, which never happens with narcissists.
By definition, they give themselves too much importance and do not have much regard for the other person. The good news though is that there are tell-tale signs which can help one identify if they are dealing with a narcissist and become aware at the earliest before too much physical or emotional damage is done. Read up and for best results!
If one feels that a person has narcissism that could be causing harm to oneself or to others around, and if you are unable to handle the situations yourself then they can seek professional help or counseling.