When the Narcissist no longer needs to use you as a source of Narcissistic Supply, because they have secured a new source, they “will discard you”. That is when the Narcissist’s Mask will come off and you will get to meet a totally different person.
The person you fell in Love with, never existed. You fell in Love with the Narcissist’s False Self, which they projected. You fell for their Mask and when that Mask slips off, it can be shocking.
UPDATE: I recently posted about why victims “could not” leave very physically abusive relationships. View that “Why Abuse Victims stayed” post and it’s video by clicking on this line. Note how the victims referred to the person who was abusing them. They were still in Love with the Fake Persona which had been presented to them during the Love Bombing phase.