The narcissist sees human relationships through the lens of possession and narcissistic supply: any relationship the narcissist has is only for the purpose of narcissistic supply, and for the "gratification" the narcissist gains from feeling they "possess" the other. For the purposes of this article, I want to focus primarily on intimate or sexual relationships, expanding on some points made elsewhere about narcissists and sex.
This sense of "possession" can extend long after the narcissist has even been in any meaningful contact with the person (the source of "narcisssistic supply") in question. The "relationship" may well be long over, as far the the "victim" is concerned, but (as Kim Wilson explains) the narcissist will never see it that way, as they never fully "let go" of the relationship.
The only way that will happen is if the "victim" can fully sever all contact with the narcissist; however, the narcissist may well have an insidious way of maintaining "contact" with this victim (for instance, through using a false identity).
"Seduce and destroy"
The narcissist sees relationships through the prism of their own "value system". What this means is that the narcissist's view of the "victim" is completely skewed in relation to reality: as the narcissist has an already inflated view of his (or her) own ego, it corresponds that anyone who is seen as a "source of supply" must also be seen in relation to what kind of supply the narcissist seeks from the "victim".
As Kim Wilson mentioned in the first of her links highlighted, the narcissists perspective on the victim also affects how the narcissist deals with the "victim": whether the narcissist considers them to be "high level" or "low level" supply.
In the context of sexual relationships, "low level" supply here may well mean that the narcissist will callously "take what he can get" (i.e. that "low quality" supply is better than no supply); in other words, the women he is involved with at this level he considers as nothing more than human playthings, to be used and discarded when he's done with them. As said in my earlier article about narcissists and sex, the narcissist will use them as a way to masturbate into someone's body. They are treated in the same way as prostitutes, except that the narcissist here, for his own (vain) reasons, prefers a game that may as well be called "seduce and destroy".
What is most important is that the narcissist is always seen as "winning" in some superficial way. Psychologically, "low level" sources of supply are somehow meant to feel grateful of the narcissist's attention (or even what he sees as "pity"); by being with them, he's somehow "helping" them altruistically by giving them his time and attention. Using his charm, he is "allowing" them to feel better about themselves for him being around them, gaining from his "reflected glory"; using his charisma, he's giving them the "opportunity" to learn from his own character to better their own (and thus, implicitly, characterise himself as a "role model" and "God-figure"). Then, when the supply source is inevitably discarded, the narcissist will callously justify this as an educational "lesson" for the victim; an experience that the victim can learn from, where ultimately the narcissist has - in an act of benevolence - given their victim their freedom back (as a jailer would a prisoner), once they're no longer of any use to him.
Thus from this twisted and perverse logic, the narcissist sees his usage of "low-level" sources of supply as practically a form of altruism, where's he's "doing them a favour" by deigning his victims with his time and attention.
"Idealise and devalue"
When it comes to what the narcissist would characterise as a "high level" supply source, the "game" is very different, for the "stakes" are considered far higher to the narcissist.
In this case, the relationship takes on a much deeper, much more serious, meaning. If the victim is a "high level" supply source, it follows that they must have been worthy of a great deal of the narcissist's praise and devotion; to be worthy of such elevated standing is "high praise" indeed from the narcissist. This is where the "idealisation" of the victim comes from. The narcissist is not truly in love with the person themselves, but the "idea" of what the person represents.
In order for the narcissist to consider a relationship with "high level" source of supply, they must somehow either be considered a "match" (i.e. "soul mate") for their own ego in some manner, or (even more drastically) a kind of fantastical version of an ideal partner. In either case, the victim is in for a very rough time.
The problem here is that the narcissist is never truly happy for long, even in a relationship with what he sees as his "perfect" life partner. Even when they are "happy", they are insecure.
In Oliver Stone's brilliant biopic of Richard Nixon (played by Anthony Hopkins), he is portrayed as someone who could never be truly happy, even when he seemed to have everything he wanted; in the film, this was something his wife knew about her husband all too well. It was this neurotic aspect to Nixon's character - i.e. his inability to never feel secure, which fueled his paranoia - that was seen as the driving force to his destructive fall.
In relationships, the narcissist's incessant insecurity is what feeds a self-destructive cycle. The constant need for "validation" from the narcissistic supply source would make any ordinary person go crazy; the narcissist's relationship with their "idealised" partner becomes increasingly possessive and suffocating. This would then make the victimised partner feel the need for more space, resulting in the narcissist becoming paranoid about their partner's activities, resulting in a greater need for control over their partner, leading to greater alienation, and so on, until the inevitable break. This is just a summary of one possible outcome, but one that the narcissist would make inevitable through their destructive behaviour; the only question is if the "idealised" victim takes the initiative to sever the relationship.
Another possibility is that the supreme narcissist decides he can "have his cake and eat it": he may decide that now he "possesses" the ideal partner, he can "play the field" as well. In this "rationalisation", he finds the best way to "prove his manhood" is by cheating on the very person he deems his "ideal" partner. By the same "rationalisation", this acts as a true "test of loyalty" towards their partner; if their partner "fails" the test by disapproving of his conduct, then this simply proves how the "idealised" partner had been somehow "unworthy" of the narcissist's attention all along (the "devaluation", which we'll talk more about in a moment). So in this sense, the narcissists desire to "test" the person he is meant to idealise would be another sign of his inherent insecurity. This "testing" behaviour could take many forms; cheating, is simply an extreme example.
As there is little way that the victim could live up to the narcissist's "fantasy image", the victim is bound to ultimately "disappoint" the narcissist in some way. So cheating, or finding various ways of making the victim seem less and less in control of their lives, is how the narcissist sees himself as the "winner". There are many others (for instance, see my summary of "cerebral narcissists").
When the narcissist's conduct results in the end of the relationship, we come to the "devaluation" stage (of course, if the narcissist's "affection" is unrequited from the start then this opens up another can of worms entirely).
Now that the narcissist can no longer physically "possess" the victim, the "possession" must take other, more insidious, forms. If the victim is a "high level" supply source, the narcissist will do whatever they can to maintain that supply. How this is achieved depends on the circumstances.
First of all, the narcissist may try to maintain a platonic relationship. This is (in some ways) perhaps the least "destructive" option available, but the fact that the narcissist has that link to the victim feeds part of the narcissist's sense of control (i.e. supply), and gives no comfort to the victim. This is especially true if the narcissist shares, or infiltrates, the victim's social circle. By being part of the victim's social circle, the narcissist can feel they have psychological control over how the victim is perceived. This is one example of how the narcissist uses insidious methods to maintain narcissistic supply even after the relationship is over. The narcissist can then "devalue" the victim among their peers at will.
Failing this, the narcissist will create supply from the victim in other, more destructive, ways. A real-life example of this is the proliferation of so-called "revenge porn": if the narcissist cannot psychologically "devalue" the victim through their relationship with their peers, they will do so in more direct, and humiliating, way. The narcissist fundamentally doesn't care if the source of narcissistic supply loves or hates them for their actions; what matters is that they're thinking about them. Supply is supply; attention is attention, whether positive or negative. That's all the narcissist cares about, when it boils down to it: as they see it, the more the narcissist does to become part of the victim's thoughts, the better. And by this point, the more "devalued" the victim is, the higher the narcissist rates his own status in comparison to them. This is the ultimate form of "victory".
A dark obsession
What happens when a narcissist's attention is not returned? This is the destructive, dark path that can lead to obsession: in real-life, this is the psychology of the "stalker". As mentioned earlier, narcissists "idealise" their victim when they are identified as a "high level" supply source. They live in their own fantastical world, and when they have identified the victim as a "high level" source of supply, they become an integral part to the fulfillment of that "idealised" fantasy: this can then become a dominant feature of their life.
In the case of an obsessive narcissist (e.g. a "stalker"), the levels of control seen by other narcissists in relationships are taken to another level: the need to know about their victim's daily movements are in itself a form of "control", even if from afar, and without the victim knowing about it. As said mentioned before, when it boils down to it, the narcissist doesn't care if he is "loved" by the victim, only that the victim is somehow controlled by him. This idea of "control" is the source of the supply. A "stalker" takes this to dangerous level: as we know from real-life examples, stalkers have psychologically tormented their victims for years, through to causing actual physical harm to them.
In yet other circumstances, the logic of the narcissist can become so twisted around to fit their own narrative that the fact that the victim does not know about the existence of the "stalker" may be seen as a form of "victory". In other words, the "stalker" here enjoys their anonymity while knowing everything about their victim's life: to the narcissist, this is yet another form of "victory".
As can be seen from these examples, the narcissist's use of relationships is all about power and the need to "win" over the other. The only question is how this is manifested.