Let me make a crazy guess. They returned to that gate later, MC fell into some hole, and it led him back to x years ago, when ML was very young and blind. And then he returned the same way. The only thing that confuses me about this version is why he lied about his name to ML and didn't tell him anything... like, so as not to ruin the future?
SPOILER
<spoiler>
The theory is actually correct, he will voluntarily fall into that crevice because he wants to drag down the main villain with him and help the ML. This is because he's been crossing the limit of his power to handle another character that's going to go berserk and fight the main villain. After he falls into the crevice, he will meet the ML when the ML was a kid.
</spoiler>
if you want to know who will help him going back, it his <spoiler> sister </spoiler>











I wanted to ask if it was Mincheol. But my second thought was: Taeha's brother. If it was Taeha, what kind of magic is this, that he's always on time when you need him, tsk tsk.