LOL! Lots of therapy!
However this is a very short summary of what I've learned so far - the core of it comes when it's a parent that has NPD because it gets in the way of individuation which is key to healthy adulthood. Basically narcissistic parents teach their children that they are merely an extension of them (the parents) without a true right to exist (not literally but as themselves). So we grow up as a mirror of the parent. The emotional abuse is very conniving and subtle as well...so hard to explain. There is a lot of gaslighting and the abuse is cleverly disguised as "concern"...the child is often labeled crazy by the parent to not only belittle them but keep them in place. For me first I had to recognize the truth (it's hard with all the gaslighting and crazymaking), then I had to go back and feel all of those childhood feelings I numbed to survive and then I had to mourn the parent I never had (and may never have - many NPDs are untreatable)...I'm still in this process which is hard. The final step which I'm working on as well is about developing my own identity (individuation), dealing with my mother in a different way (many have to go NC with NPDs) and learning that the childhood survival techniques don't work for me today, and working to stop the cycle (to see if any of the tendencies passed down to me).
The key for me was admitting I truly did feel I didn't have a right to exist AS MYSELF (not literally - haha). That opened the flood.
There are a lot of good recovery books on this most of the ones I use are for daughters of NPD mothers but their are books for sons too and the keys are the same.
Here is a great article that almost described some of my childhood word for word:
https://sites.google.com/site/harpyschild/ (this is specifically mothers but I think it relates to all NPD parents just adjust a bit)
My therapist also had me read:
Trapped in the Mirror by Elan Golomb...this one is written by a Phd so beware...but it was worth it for me.
The article really gets at it though - the emotional abuse is so strangling and hard to shake even as adults.
I'm sure I rambled and this probably didn't help much but I thought I'd take a stab at it.
Narcissistic parents lack empathy which is the most damaging thing.