something my parents learned from their therapist in dealing with my brother -- his specific diagnosis wasn't exactly malignant narcissist, but he was dominating their lives in unpleasant ways and impossible to deal with due to his mental issues.
establish clear boundaries in your own life. no matter how big an impact this guy is having on important issues, make time and space that has absolutely nothing to do with this guy.
talk about him and the issues he causes when you need to, but have that safe toxicity-free time and/or place as well.
so you're talking about him all the time, except date night, dinner time, specifically 7pm-9pm, any saturday, whatever time and rules work. maybe the tv room is the safe room where the toxic guy is not mentioned. when and where that topic and that name are simply not allowed to be brought up. just steer the conversation away from that topic. if he was at an event or meeting you want to talk about, fine, just talk everything else about the event besides him. if you realize you have to say something about him, make a note to talk later when you're no longer in the safe time or safe room.
this reminds you that you have a life outside of this person, things he cannot touch or ruin our interfere with, things that have nothing to do with him. it helped my parents reconnect with each other and the rest of their family and friends and hobbies and so on and keep their one problem child in perspective. eventually they got to a point where they realized that they were adult with a happy life with a lot of connections and a lot going on and one of three children was being a pain in the ass. it didn't make dealing with that one child any easier, but it helped them realize they didn't need to let it dominate their lives, and it didn't need to consume any more time than they were willing to give it, which wasn't much as they had happier and more productive ways to spend their time and energy.
don't know if this is anything like what you are looking for but hope it helps give you perspective if nothing else.
in terms of dealing with him in terms of custody battles and so on, temporary restraining orders are your friend, and audio/video evidence of rotten behavior, especially if it violates a TRO is great. if that's relevant to your situation, installing a few cameras might be a great investment, make sure they're properly positioned and recording everything.
also remember that the judge is always going to prioritize the child's interests (or what they officially think is in the child's interests) so evidence of him being an major ass to *you* is not necessarily going to make an impression on a judge, but him showing disregard for his child's safety would be a big deal.
hope it all works out!
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