The simple answer is it's whatever the Narcissist wants it to be. In essence whatever mask they can use to hide the insecure and damaged part of themselves to obtain the narcissistic supplies they need to support an inflated view of themselves.
The more complex answer is that the false Self is a protection mechanism against attack from the outside world. The Narcissist may suspect that something is wrong in their make up but they choose not to investigate the source of their insecurities and fears, they deny their feelings because it would mean they are not perfect. They don't want others to see their defects because if they are pointed out it casts doubt on the grandiose image they have of themselves. Hence the development of a false Self that they and others can respect, admire and "love". This is what their childhood has taught them, if they always behave as expected people will perceive them as special. If they show them their faults they are not special and others will deny them their respect, admiration and "love".
Narcissistic people will do whatever they can to ensure people see them as they see themselves i.e. special. Some focus on materialistic things ensuring they are seen in the best clothes at the right places and driving new or expensive cars etc. even if they can't afford to live that way. Some Narcissists in particular will focus on the perfection of their bodies.They will want to impress others with stories of their accomplishments; new jobs, wage rises, expensive holidays, their sexual conquests etc. (Sometimes it may be apparent that these accomplishments are not as substantial as they have been made out to be or are not attributable to their own efforts. Lies and distortions of the truth are not unusual from the lips of a Narcissist.)
On a personal interaction level they may offer to put themselves out to help you in your work, treat you like a Princess/Prince, taking you out and buying you gifts, complimenting you, making seemingly creative one off gestures to show they care or have thought about what you would like, treating you as a valued friend, spending lots of time with you etc. They make you feel special and at this point you are special to them, not for who you are but because they see you as an extension of themselves. This view of you is known as "Inflation". (On the part of the Narcissist all these lovely gestures are usually undertaken on a shallow level and if you are emotionally invested it is unlikely that you are going to notice until it's too late - even if they have given you good warning that they're not always this nice).
Narcissistic people do not make these gestures selflessly; they behave this way to feel good about themselves which is accomplished through how you feel about them. The positive feelings (and at times negative ones) that they are in essence trying to extract from you are a bid to confirm that they are important or special. These feelings they extract from you are known as narcissistic supplies. A source of these narcissistic supplies is what Vaknin has termed a "Narcissistic Supply Source" (NSS).
The Narcissist doesn't have the emotional reserves to pull off this fašade indefinitely - it is hard work.
Now before I give you the impression that narcissistic people are bad to the core, purposefully out to hurt others and should be segregated from society (which is not my intention) I would like to point out that they may or may not be consciously aware that their entire lives are geared to securing narcissistic supplies. They may know they like to be desired, admired, adored and respected but unless they have had some form of counselling or come from a psychological background it is unlikely that they understand their impulses, feelings or behaviours. Often they don't want to understand because they are just happy to exist, through your eyes.
A Narcissists charming behaviour lasts as long as they get what they want from you i.e. that you provide evidence to confirm their view of themselves as special. In any relationship where people spend a lot of time together it is normal for the parties involved to notice the others bad habits and behaviours. To some degree it is normal behaviour for people to point these bad habits and behaviours out to the other person in order to try and improve the relationship. Narcissists do not react kindly to this, they see it as an attack on their perfection. As a result of this their behaviour starts to change. The lovely person you know will start to disappear as the damaged part of them emerges, the things they do to impress you will become fewer and farther between, you will start to feel less and less special to them as they start to treat you badly and show their frustrations (overtly or covertly) at your inability to meet their needs more frequently. This is known as your "Devaluation".
Vaknin maintains that this process of devaluation can occur when the Narcissist becomes bored or used to the way you function as a NSS.
During periods of frustration which are brought about as a result of their needs not being met narcissistic people will certainly make you aware of your inability to meet these needs. They will either do so overtly; raging, abandoning, publicly humiliating you, belittling your abilities and possibly becoming abusive. Or they will do it covertly; giving you the silent treatment, sulking, discussing things that they know will upset you or behaving in ways they know you find hurtful regardless of if they are related to the frustration you have "caused" or not.
If their frustration stems from their own inability to meet their wants and needs e.g. not getting a promotion, loosing at sport, receiving criticism for work they have done etc. the frustration is usually transferred onto the people they interact with (often their nearest and dearest) or it is channeled into harmful activities such as excessive drinking, drugs, gambling, driving fast etc. In "transference" what the Narcissist is in effect doing is taking their feelings of "I'm bad/inadequate/guilty" and passing them on to you because they don't want to cope with anything that shatters their good image.
Maybe at this point you will try to work harder to make them happier; you will start to question the things that you have done wrong to bring this change about and try and make amends. Maybe they will start to manipulate your emotions to extract more narcissistic supplies from you even if that's through intimidation and fear. They may even tell you the truth, tell you that they are no good for you and how badly they behave. Maybe you'll jump to their defence telling them they are valuable with renewed hope that there is something alive/good inside them. Before long you won't know what's real/the truth and what's not. Years may pass and you will wonder when the person you first met is going to come back. If you are prone to loosing yourself in relationships it's likely that by the time you've decided you've had enough (or you've been dumped by the Narcissist as a NSS) you will already have lost sight of the best parts of yourself and taken on a few of their more interesting character traits.
When the end comes it doesn't arrive quickly. The Narcissist may come back for more when other NSSs aren't in ready supply or not doing their jobs properly. They may even enlist the aid of their friends to bring you back to the fold or intimidate you. Or maybe you will try to go back to them because you miss their presence in your life (which is usually the result of a loss of adrenaline that was previously generated by the highs and lows that interactions with Narcissists bring). You're unlikely to get any closure on the basis that you to and fro or because there isn't going to be a happy ending where you're going to get the answers you need to feel happy about what happened in order to be able to put a lid on it.
There are a few things you can console yourself with, firstly, and most importantly you will have people you can turn to. Your friends, your family (if they're not part of the reason that you've accepted this type of Narcissistic behaviour for so long) and your Doctor or a Counsellor. Even people who cry in the street are asked "Are you ok?" if they stand still for long enough, I've seen it (not that I suggest you trust any stranger you meet - choose your confidants wisely I made a mistake or two myself).
Secondly, the Narcissist isn't going to be spending time worrying about how you're feeling or analysing their part in what went wrong in your "relationship". This is because they won't think they've done anything wrong and they don't like to dwell on any negative emotions. In fact it's likely they will feel a sense of relief that they no longer have to work so hard at keeping up appearances. They'll just go and find another NSS to replace you, if they're good looking or successful it won't take them long so rest easy for them. Talk about it out loud, analyse it till you're blue in the face, anything that helps you to process what's happened and learn from it.
Thirdly, as far as I can see only five sets of people can maintain a long term "relationship" with a Narcissist; an honest to goodness Saint (how many of those have you met?), a good Therapist, "fair-weather" friends (who are often as shallow as the Narcissist is), family members on whom the Narcissist is dependant for narcissistic supplies and those who don't mind (even enjoy) being a NSS because they're probably not aware they have absolutely no security that the Narcissist will admire, respect or love them back long term. So try not to be too hard on yourself if you've taken all you can stand.
Finally, things will get better. It may not be quick in fact it may be hell on earth for some time but like any loss or death of a loved one it will get easier for you to cope and you'll start to feel good about yourself again.