In essence, after enduring several days of fasting and progressing in his objectives, Hazen goes to have a drink with Yan and talks about his past, which triggers the flashback and ends in Yan's memories being sealed.
It's obvious where the author was going with this, but the execution is poor and not well though out, which obviously left many readers confused.
This will become a plot device for much later in the story, when Yan and Hazen will be in conflict, only for those memories to be released once Yan prevails or they go their separate ways due to misunderstandings. Wish it had been done better though...
It's not confusing though? If you reread the previous chapters properly, it makes perfect sense, and this whole plot point has been teased several times throughout the series so far, it wasn't one that was brought up out of nowhere. From the beginning they had already been setting this conflict up, even the part where Yan was supposed to go her own seperate path when Leifa confronted him about her being his replacement.
This person's take is a good example of what the author was intending for readers to take away:
The story establishes that Hazen's previous life was a famous long-lived powerful wizard. The wizard in this chapter is similarly famous, powerful, old enough to have an older teen/adult daughter, and even shares Hazen's face. The boy Ash is shown becoming friends with the wizard's daughter for a short-lived time before she quickly falls terminally ill. Becoming desperate, Ash delved into forbidden arts to save her while the Wizard is preoccupied with his own research into a cure. However, Ash's acts corrupt him into an immortal monster and Ash abandons his humanity, the daughter passes away uncured, and we know the wizard ultimately failed to kill the non-humanoid Ash and died. Page 17 even has him slipping up due to drinking and switch from the third-person Wizard to first-person pronouns.
I think since we're shown Ash transformed into some snake thing, the other wizard character is someone else. Maybe even responsible for the illness, whispered about the immortality cure to Ash, etc.
I also think the ambiguity on who the MC is in the story is also on purpose, and the general ambiguity over our MC's origins are meant to be that way since at this point in the story, the author doesn't want us to know what is up yet, and is simple teasing the future plot points. With this ambiguity, it builds suspense, curiosity, and discussion over MC's true identity and his future plans, and is much better written than a shitty isekai outright telling you exactly who the MC was. It's a mystery meant to be theorised on by the reader.
If I had to give my take, I'm assuming he might be going his seperate way back to his old continent to tie up the loose end by killing Ash who he failed to kill in his previous life, using the knowledge of the new magic system and the resources he gathered in the current continent, leaving Yan to finish his job of getting rid of corruption, though it wouldn't explain why they would be on opposing paths. If he is Ash, then maybe after dying to the wizard, he is going back to finish the job of reviving the daughter, where he would be in conflict with Yan since I don't think she would approve of black magic. Although chapter 21 heavily implies he's the wizard though so it would be an insane plot twist to me if it turned out he was Ash.
