I'm not sure if it's a solo story or an arc in a QT novel. The ML is the Leader of the Devil cultivators who was head-over-heels obsessed with a white lotus beauty he picked up (after killing off the beauty's clan). In the end, he was betrayed by the white lotus he was cultivating to be his Dao companion. The white lotus cuckolded him with the world's 'protagonist' by offering up the ML's treasure to the 'protagonist'. Then, comes the transmigrated MC after the plot revelation. He is a small disciple picked up by the ML. Basically a lad who was as obsessed with the Leader as much as the Leader was obsessed with the white lotus. Everytime the Leader goes off the rails after every being treated coldly by the white lotus, the MC will be the only one to keep the ML's temper at bay. Even though his cultivation was low, he was able to guard by the ML's side as early as he was able to eliminate all other candidates in a tournament. I read this one two years ago, then waited for updates, instead of MTLing~. Sadly, I lost my bookmark. I implore all fujo and fudan senpais to rescue thy kouhai. Onegai~