The spirit of a creature trapped in amber cannot escape to it’s afterlife / next incarnation / whatever.
Such trapped spirits can be contacted by those who know the right rituals and magical power derived from them – typically the power to control one individual of that species.
Can by quite powerful if the creature is a colony insect (ant, bee, wasp, etc) – simply control the queen and you command the whole colony.
Real world scientists have accidentally opened a tear in the fabric of reality, transporting their laboratory into the game world. Unable to repair and/or refuel their equipment in this (to their minds) primitive world they are now trapped.
The local band of 'nasties' (goblins / orcs / whatever) lives in relative peace with the local population.
Along come the PCs and go through their usual heroic monster bashing routine, wiping out the nasties and pinching all their stuff, then continue on their way.
Problem is, they don't kill ALL he nasties. The survivors want revenge and, after spending a few months recovering, start to take it in their own inimitable style (which is not unlike that of the PCs come to think of it).
Next time the PCs are in the area they find themselves VERY unpopular with the townsfolk.
A plague has hit the local area.
In humans it affects only the most vulnerable, the children and the very old, and even than it's little more than a summer cold. 24 hours of sniffles and then it's gone, barely noticeable really.
To sheep however it is fatal and the whole economy of the area is in serious trouble. If this keeps up the area could well be facing famine.
Somebody (enter the PCs) must find the rare herbs needed to make a cure.
The air had grown chill the minute they descended into the strange valley, which was unmarked on any of their maps. It was so strange here, devoid of animal life and completely silent. The horses were nervous the entire journey through the vale. As they set about to collect firewood for the campfire they could hear their own voices as dim echoes through the eerie silence.
The food didn't taste anything that evening and their sleep was cold and troubled by nightmares. While they are clearing camp the next morning, one of them stumbles over a piece of stone jutting out of the ground nearby the horses. On closer inspection there seems to be runes engraved into the polished surface. The symbols true meaning is no more known among mere mortals and if they decide to dig deeper, they will discover that it is an ancient altar buried within the soil.
Any historically oriented party member will recognize the largest symbol to be the insignia of the powerful warlock who ruled this realm several centuries ago. At their departure from the area, something will seem amiss with one of the party members and all will remember the stories of the warlock's thousand curses.