While delusion is more of an internal process, lying and denial is often in the context of other people.
Regular people deal with their problems by themselves, internally. Or they discuss it in a very private setting: in therapy or among very close, healthy people. Narcissists don’t have people like that in their life and are not really interested in actually resolving anything or being introspective.
Narcissists simply want to know that they are in the right. For that, they need other people’s false validation to regulate their shaky self-esteem. They need to find people who would agree with them. And in order for others to agree with them, these other people either need to be terribly unhealthy and unable to recognize their toxic tendencies, or the narcissist needs to lie and present a different story than what is actually true.
Here, they tend to flip the roles where they are good, noble, caring, virtuous and the other person is evil, cruel, selfish, and immoral. Which brings us to the next point….