This is such an important thread and topic and I think also helps us contextualize what we are seeing. It is no coincidence that our former spouses' behavior is being described on forums all over the internet!
For everyone on this thread, I really recommend to everyone here Shahida Arabi's books on narcissism. They helped me so much to understand the behavior patterns but also offer a genuine roadmap to healing and even post-recovery. Really smart, articulate, insightful, at times unconventional, yet also warm and compassionate. She also has free articles and materials online.
I believe that MLC is related to the narcissism epidemic and that both are biology based. I think the connections we often make to childhood is just same hereditary mental illness, spectrum disorder, or susceptibility. I think this explains why our spouses often reject their parents' behavior only to copy it to the letter as we look on incredulously, even lacking the basic self awareness that they are repeating it.
My ex had a series of unusual fevers leading up to this, along with ongoing migraines. I talked to a neurologist at one point and she said these were a huge red flag. He has changes to his physiology that cannot be explained simply by psychology. Even our "full moon alerts" are a big indicator that this is not simply childhood trauma.
(In fact speaking to childhood trauma, I also want to add that some of the most deeply compassionate people who have helped my son and me through this had traumatic childhoods, which lent them wisdom and deep understanding.)
I strongly believe both MLC and NPD are connected to damage to the frontotemporal lobe. Whether this is viral, fungal, or some other issue I can't say. I read a fascinating article about an Israeli researcher who felt that narcissism was adult-onset autism. It made sense to me as we can clearly see our former spouses quickly "regress," almost "overnight," in much the same way we read about infants suddenly regressing.
My feeling about my ex and who he "really" was varies widely. At times I think he was always like this, but then I have had some honest moments in my heart where I remembered how funny he was, how his eyes were sparkly, and I think that if there was a period where he was "faking" it — even if it was longer than I realized at the time — perhaps he was masking the symptoms in the same way I might if I slowly realized I couldn't feel things the way that I used to. He probably was always a little "on the spectrum" of narcissism (I would have just called it a little inconsiderate), but I don't always think it was as sinister as some sites on covert narcissism would lead you to to believe.
I think that there is a tipping point, where MLCers lose both empathy and self awareness, and this is when we see bomb drop. If you read about frontotemporal lobe impairment and damage in narcissism and psychopathy you can get a clearer idea of why those afflicted with all of these conditions (or this one spectrum condition, we don't know) suddenly becomes as if a pattern.