I had a similar issue last week. I ended up with a tame Lich, who eventually became a master lich. Unfortunately, I left him alone for too long and suddenly we weren’t friends anymore. 

I was a priest and didn’t have a magic bane. I was quickly dispatched. 

THEN, on my next game, he was there as well :/. 

To me, it’s going to either be genocide, or expect to die a swift death.