I suppose it is a matter of subtlety. If I know a player wants to get rich and I drop a hint that the local dungeon is full of gold they will probably go there on their own, where as if I say "Look, the adventure is in the dungeon. If you want to play tonight your characters better get their butts over there," it is pretty blatant railroading.
Both end up with the PCs running the dungeon, but the first doesn't feel like I am taking away their agency.
I suppose I can give more info, although most forum readers are like PCs, too much backstory scares them away and kills the thread. Some of this might seem familiar for those of you who have been reading my threads in the past.
Long story Short;
Current campaign is set several centuries after a previous campaign.
After the old PCs killed the BBEG his evil kingdom was left a desolate wasteland with no clear ruler. (Think LoTR if after Sauron was destroyed the surviving Nazgul each declared themselves rulers of Mordor and made the orcs fight amongst one another).
A young hero who wanted to follow in the retired PCs footsteps led a crusade into the wasteland to cleanse it and destroy the remnants of the evil forces.
The crusade disappeared and no one knows what happened to it.
Several hundred years passed and the area is no longer a wasteland. The PCs grew up in a small village in this area.
They discover the ruins of an ancient keep. Inside they find that this was the last resting place of the crusade and once a stronghold of evil. The lich who ruled the stronghold was destroyed and his soul imprisoned beneath the tower, but the crusaders, lacking the provisions to return or the strength to continue holed up in here and died, eventually resorting to cannibalism and dark magic. Their ghosts haunt the tower.
PCs explore the tower, adventure over.
Next adventure the enemy of the current campaign has a massive army and the PCs village is right in its path. I want the PCs to somehow recruit the ghosts that inhabit the nearby tower to aid them, once again similar to the Army of the Dead in Lord of the Rings, both saving the town and redeeming the lost souls.
The problem is I have no idea how the PCs would communicate, let alone control, the ghosts on a large scale. The only idea I can think of it awakening the lich that is imprisoned beneath the tower, but the problem is getting the PCs to have that idea (particularly at the right time, if they just happen to awaken him when they first come to the tower they will all end up dead).
So, one of the PCs is a child priest who is believed to be the reincarnation of a saint. The advice was to have the saint he is a reincarnation of be the one who led the crusade. Which is a good idea but...
I already know who led the crusade and have a fairly detailed backstory for her
It was a secular rather than a religious crusade
The order he is a member of didn't exist when the crusade was lost
The crusade was lost, and what happened to it was a mystery. If the leader had gone on to found an order of monks and then have a line of incarnations it would imply that someone survived the crusade and knew what happened to it, which deflates the mystery entirely.
And there were a couple other problems that I can't quite recall atm.
So yeah, there's the story, as brief as I can give it. Let me know if you have any thoughts or advice or need any more info.
Thanks!