I am currently at an impasse with my fantasy novel project. I am unsure whether or not to split the novel's narrative between a "present-day" storyline focusing on my protagonist and an extended flashback following his mentor. The present-day storyline follows the protagonist's attempts to escape (with mentor and company) from prison and their perilous voyage across enemy territory to freedom. The mentor's flashback would follow his involvement with a sect of sorcerers who have banded together to wage a resistance campaign against the empire which has hunted sorcerers for centuries, culminating with his eventual imprisonment. Option 1: Split perspective +Story doesn't become bogged down with grimdark prison cruelty for the entire first act +Mentor character is a lot more fleshed out and fosters more audience investment +Opportunities for character parallels between protagonist and mentor -Detracts from the reader being engrossed in the protagonists emotional journey -Time shifting can be disorientating Option 2: No Split +Plot is linear and concise +Clear focus and engrossment with the protagonist's arc -Prison story could drag on until the escape scene -Mentor at risk of becoming a one dimensional character
Hm, in my opinion having a flashback and a present-time storyline really should strongly relate to each other in some way. An extended story just ending up where the mentor starts in the present-day story may not be strong enough. It is the sort of thing that can depend on execution, but from what you've described I think it might be best to only write the protag's story, with the option of perhaps making the mentor's story a prequel novel/la. Or possibly even just write the mentor's story and then the protaginist's as a sequel to that.
If the events described are harrowing, reading the account may feel harrowing too, which wouldn't be much fun for the reader. If the mentor makes odd decisions such that the reader thinks the character is badly written, it may be necessary to include backstory to explain why the mentor thinks in the way he does.
Best of both worlds, you could always have the character in question reference something as the reason for their actions. This illudes to events from the past and allows for a slight character building moment without interupting the flow of the story.