"What is a narcissistic injury?"
Whatever you internalize from his/her conduct as your fault. As guilt tripping and projecting is apart of abuse and manipulation used by abusers to weaken your sense of self and resolve.
If you understand he/she owns their own conduct and self destructive behavior and remove them from the realm of your feelings the injury belongs to them and not you.
Best wishes
Narcissistic injury is not what the narcissist does to you, but what they think you've done to them. Criticizing the narcissists causes HUGE narcissistic injury. Ignoring a narcissist causes them injury. Failing to see them as Gods...ect. Eventually everyone will cause a narcissistic injury to a narcissist and they will devalue you.