Coming here for this because it is amazing how often you all can identify a story, book, series from an extremely limited description. I have 2 series I am looking for that I read a LOOOONG time ago (30+ yrs). I have no clue of the authors or series names, just a vague description of the overarching plot of one series and a starting point of the first book of the other series.
First series was a YA fantasy series I think (I read it in middle school and the books were avail in the school library). The MC was a young (teen? early 20's?) Prince who somehow (cant remember details) gets sent on a quest to find 7 mystical keys in order to save the people of his kingdom. He is accompanied by another (friend? guard? relative?). I believe there were 4 books in the series (might be wrong) and the books detailed the hardships of questing for the first couple of keys then I think the story skipped to going after the final key. When the story starts, the Prince is kind of spoiled and definitely "soft" and unused to hardships. By the end of the story, he has done a 180 turnaround and, IIRC, either sends his companion back home or leaves him to heal from a wound while he goes on alone. I do not remember what the "threat" to the kingdom is that the quest is the solution for, nor do I remember how the story ends. I do remember later when I read Jack L. Chalker's Rings of the Master series that the questing/flow of the storyline was similar.
Second series I have even less of a description. I do not remember how many books were in the series (at least 4 but I am thinking most likely 5 or 6) nor what the overarching plot was, just bits and pieces of the story from several of the books. The story starts with the MC going on an expedition in the jungle (I think in Africa, though it may be in India or South America). I think the expedition was following/looking for a previous expedition that had been lost. Time period was mid 1800's I believe to start, but time was rather "flexible" throughout the series. Somehow the expedition (or maybe it was just the MC?) was swept away down a river and (swallowed by whirlpool? swept into a cavern? cant remember for sure which). This resulted in being trapped in an alternate/twisted world/dimension/area? that served as a prison of sorts. Both the first expedition and the second expedition were seeking something (again cant remember what) and the prison was holding all the people throughout history that have gone looking for it. Yes, the prison exists outside of time (one of the reasons time is so "flexible" throughout the story). The prison is actually only the very first step in the quest for whatever it is they are searching for. I remember the MC found members of the original expedition in the prison...and the majority of the first book is spent escaping the prison to get to the next step of the quest. I remember that throughout the series, the MC learned that he couldn't trust anyone because he kept being betrayed over and over again. And every time I thought I had a handle on what was going on in the story, who the players were, what side they were on, etc, the entire thing would be thrown in the air, everything mixed up and I would have to start all over trying to piece together some understanding of what the hell was going on in the story. I also seem to remember at some point in the story a mystical train that played a large part.
While I have said a lot, I know that the actual details I have given are very few. But if there is any place I can turn to that might be able to help me identify these stories, I know that this forum is it. I have constantly been amazed at how often a story is correctly identified with even less info than I have given. Thank You all for the assistance you give those of us who get bugged out trying to remember. :-)