it's shown/mentioned in one of the chapters (forgot which one) and ch.49 that gu lang blinded him by accident due to the mist/gas in the mountains that made gu lang hallucinate that murong was zheng yufei :'(
spoiler alert in case!it's shown/mentioned in one of the chapters (forgot which one) and ch.49 that gu lang blinded him by accident due to the mist/gas in the mountains that made gu lang hallucinate that murong... LG ✰
just had time to check haha it's explained in ch.36~
how did he turn blind?