From what I can gather he's alluding to something about Mimit that would allow the Demon Lord to finally conquer the world, but doesn't outright say what. That and nostalgia for who she used to be.
bael points out the demon lord has always been soft on her, mimit saids that a survivor from raza claimed they were destroyed by one monster, she identifies that one monster as the demon lord, which means he personally destroyed raza to get her the grimoires
My guess is that Mimit probably got too close to actually winning the war, and whatever gods there are had to directly step in and smack her down to maintain the stalemate, likely either causing her to repress the memories herself or having the memories sealed, but the trauma itself is still intact