Not a fully fleshed out idea, yet, but this came to mind:

The 9th gate Saga

A banshee/spirit shows to tell of her plight.. needing adventurers to help "free" her sisters from undeath so that she may herself finally rest.

To get to each requires a key.. a key that can only be found from the previous undead being.. each being requires something special to kill them, to put them to rest
- some aspect of their lives - personal info used to find out what is needed to "set them free/put them to rest"
- some different weapon type

in order to unlock the last gate requires all keys.. everyone who has helped up to that point will have designated a key-holder.. they can choose to release the last one, leave her for eternity, etc

for example.. one must be defeated by a weapon containing a firestone
one must be defeated by a mithglin sword
etc


I literally dreamed it up last night.. I apparently play too often