If I were making this decision, and none of the players had read the novel, then I would consider both the issue of railroading, and the issue of spoiling the book for them. If it were me, I would prefer to spark their interest in reading it. That puts me on the side of creating a path for the characters that is unrelated to the story told in the book(s).
When in a situation where I will be running settings tied to films, or books, or that have invasive metaplots (Star Wars, Neverwhere, Serenity, oWoD, etc), I have two approaches:
Approach 1: Normal Workload (Just using the setting)
I try to run things the same way I would run a game without a strong primary story: that is, I would put the focus on the story of the PCs as if it were the primary tale. The story from the novel or film is placed in the background as extra detail. The events in the game may or may not ever involve the events or characters of the original source material - and I would work to keep them separate if at all possible.
The goal of this approach is to create unique adventures in an established setting that all or part of the group enjoys, without worrying about existing story lines. This can be done by simply ignoring/deleting the original story, by avoiding it by setting your stories in another time or place within the setting, or by creating unrelated plot threads with no crossover with the original story and making them the focus of the campaign.
Approach 2: High Workload (Creating subtle links to the Original Story)
If I have the time, or motivation from the players to allow their characters to get involved in the primary plots from the original source, I like to remember that historical records and memory are very selective. That perspective is the tool that I use to create subplots and important interactions with the main events, which do not threaten or change the continuity of those events.
This sort of thing involves approaches such as providing roles for the PCs which make the tasks of the characters from the original source material possible. Without the PCs' actions, the course of events in the source become impossible.
example: An information gathering quest for beginning characters
An advisor to a character from the books reveals information about the enemy, a prophecy, etc in the original storyline. As I am looking for neglected or glossed over parts of the story to flesh out, this sort of scene might catch my eye as an effective way to introduce starting characters, and enable the players (who may not have any familiarity with the sources) to establish an understanding of the setting and culture being portrayed in the campaign. To make use of it, I would:
- Create a plot where that advisor had recruited one of my NPCs to uncover information in an earlier time period.
- That NPC would then select the PCs and send them on a dangerous journey to collect that information
- Use this to establish the skills and personalities of the characters, connect them in a very tangential way to a peripheral character in the original source, while at the same time making them a vital, but invisible part of the original tale
Doing this for the course of the series, will take a great deal of familiarity with the original work, and a lot of planning to find ways to have the characters support the main plots of the source material without ever being 'deemed by history' to be a major character in the story.
The goal of this approach is that when the players read the books, they can get an extra smile and surprise when they realize their hidden role in the tale that is unfolding before them.
I am the one who brought Sir Fancy-Pants the news that the Dragons had awakened! I almost died that day...
You have an active engaged player. Run with it!
I can tell you from experience that if you railroad them back onto the tracks, they are unlikely to ever be as engaged about your campaign again.
If you want to tell a story without outside input, write fiction. Dungeons & Dragons is a Role Playing game, the player should be allowed to agency to control their actions and see the world react.
It sounds like you've plotted pretty far ahead and have very specific things that you need to have happen in order to tell your story. Relying on a PC to kill a specific NPC is very risky. Players will develop morals at the oddest times throwing your plans into disarray!
Step back and do your planning from a wider scope. Instead of triggering off of the NPCs death, put a clock on the discovery of the secret mine. If it hasn't happened by September 3rd, then something bad happens. If it's discovered and the intrusion is noted, then the PC is confronted by the NPC (Your original line). If the PC accesses the mine and it isn't discovered that should be a very good outcome giving the PC a jump on the next action. (Try to avoid mandating that the PC is discovered, if precautions are taken, they should be rewarded if the dice are favorable).
By doing your plotting a level up like this you can ensure that your campaign moves ahead while giving the PC free reign to go about it as he or she wishes.
Best Answer
The answer by NautArch is great and will surely help you mitigate this problem in future campaigns, but it doesn't help much where you're at currently. Here are a few things I've personally tried as a GM.
Make New Friends
And by friends I mean enemies. More specifically, an arch nemesis. Make one of the enemies the PC has defeated before come back for revenge. Make sure it's a personal revenge directed only at one of the PCs; if you make it an arch nemesis of the group it will become the arch nemesis of the involved PC.
Repeat a few times and make sure to constantly make the enemy stronger and more involved in the main plot. Soon you have a PC that's deeply involved.
Make Old Friends
And by friends I mean friends and family. Every PC has friends and family from when they were younger, even if it's not specifically stated in the backstory.
Naturally, the lich is involved in the main plot somehow and helping out the parents is a bit more involved than first anticipated. After they're rescued, it turns out the people you rescued are doppelgangers and that the real parents are either still prisoners or safe home in the village.
Make New Memories
Anything that happens or has happened during the campaign can serve as a hook for stuff to come back and haunt the PCs. If there is nothing in particular you can draw on right now, try to set it up.
Any situation that might serve as a plot hook in a backstory can be introduced during the course of an adventure instead.
Make Old Memories
Let an event remind a PC of something that happened in the past. This gives the player a chance to flesh out the PC and add things in a natural way. Also, repressed memories can work. It can be hard to pull off properly and it cannot be used too often but it's a powerful tool.
This is great for adding needed flavor or plot hooks to a character. You can even be pretty blunt about it.
Make sure to ask follow-up questions to get a better grip on the situation.
Not only does this technique add hooks, but it also lets the players be a bigger part of the narrative and makes them more invested. It can really work wonders to make someone more engaged in the story, partly because they now get to play the exact story they want.
Just Make It So
Make the PCs be part of the narrative without any prior explanation. The explanation can either be made up on the spot or something the party has to investigate.
Whatever you do to help out the situation though, NautArchs answer is still in effect. You need to feel your players out and try to understand what it is they want with the campaign. You can even have a Session Zero in the middle of the campaign if needed. This is where we are, but where do we go from here? This is what I tried to do, but what do you want to do?
Anyway, good luck GMing. It's tricky business sometimes and much to learn along the way.