This post is a partial follow up to my previous one, I'm linking it because it does give some context on the campaign and current situation.
Basically I had this dungeon prepared (and added the monster that was suggested in this previous post), and I had this idea in mind: once they'd reach the end of the dungeon, they'd basically be on top of this mountain where this old temple's church would have been, and from there they would have seen this extremely important event. Basically, in the church, there would have been this cult leader (that they beat in the past, but managed to escape, and is now stronger) talking to this otherwordly individual (Nyarlathotep in disguise, secretly commanding the cult). The mere presence of the individual would have let them feel fear from the distance, but they would have managed to see him tear a page from a book, give his thanks to to the cult leader and told him to burn the rest, while he'd make sure to spread the rest. Then, he would have noticed the party, told the cult leader to 'not fail this time', summon a creature to help him out and then just leave by disappearing.
Basically, this temple belongs to an order who protects old secrets (and they pray to such a goddess), and Nyarlathotep used the cult to find the information within this book. What he took is an old secret (that he is aware of) in regards to this world, and his intention is to spread it to destabilise various nations, religions and so on (won't go in details to keep this short), knowing the proof of it will do that. The rest he wanted burned, because it contained some information on himself, as he is also partially involved (and of course, he does not wish to reveal information about himself).
This would have been his soft introduction, and the party would have been able to witness one of his disguises.
Combat would have come after that, and I planned for this cult leader to start burning the book if he thought he was going to lose (so towards the end of the combat), then the party would have been able to either stop him from doing so (and getting the full text), or having to stop it from burning in the upcoming turns (getting therefore a partial version of the text, some words burnt out and so on).
Issue: the wizard had Arcane eye prepared (forgot he took that), and while his arcane eye couldn't get through the dungeon's door leading into the mountain, he sent it flying above until the top, he saw the church and got the eye closer to the spot. This moment was very cool in game, and I wanted to reward the player, so I 'anticipated' the event, making them feel dread through the eye as they witnessed this individual discussing with the cult leader they knew, and taking a page from the book. He was happy that he managed to spy on them! Thing is, Nyarlathotep has blindsight, so at one point I had him look directly at the arcane eye (which had my players surprised and more fearful), I described the summoning and then him disappearing.
After that, they took a short rest, so some time passed. I used this time to have the cult leader order the cultists within the dungeon to prepare (as they would have otherwise been unaware of their coming, and so they would have found some of them doing mundane stuff like playing cards or eating), now that he knew of their arrival.
After the short rest, they entered the dungeon, and we closed the session after a battle they had with some cultists.
The issue I have is that the cult leader would have had a ton of time by now to burn the book, in my original plan he would have had to face them in the moment, making things harder.
So I'm trying to find a justification for him not having burned it already, because I did want to give the party those informations, or a way for them to restore the burnt book, but I can't find one in game as far as I'm aware (as mending wouldn't work here). The cult leader isn't even stupid, they are a spell caster with 23 int...
So, how would you do it? how can I justify them not burning the book yet, or how can I give them the information still?