I think it is around chapter 1100++
And don't know the specifics about ML using the forbidden technique as I skip many many chapters or maybe it still not revealed yet. But I think that has connection with ML soul. I remember that MC using some explosion by extinguishing her soul. In the normal route, doing that is a big no no as she will never be reincarnated. And in my theory, maybe the ML is using some part of his souls to subtitute MC extinguished souls. If there is no something to heal ML souls then his souls will never be completed...
Sorry, I already give up reading MTL as I really in need for recovery for reading MTL too much...