Elder Evils

Just like there are gods that created the multiverse, there are beings as powerful as gods (or even more powerful) that want to destroy it. These beings have been imprisoned by the gods during the creation, or have been banished to some realm outside of the multiverse. One of these beings has made contact with the troll shamans and convinced them to start worshipping it.
This Elder Evil is one particular case. In the ancient times when the gods fought against these beings, this one seemed to be one of the more powerful ones. The gods had nearly lost when someone had an interesting idea. They pulled the soul out of its body, imprisoned the soul deep beneath the earth and banished the body to some place outside of the multiverse. The soul, however, still has enough power to telepathically communicate with whatever will listen to it. And now, it has awoken after a long slumber, ready to try to escape its prison. The trolls don't know this, though. They think it's just another spirit, like all others the shamans communicate with on regular basis.
The humans are finding more and more different kinds of shattered bones and voodoo dolls scattered throughout the forest. And at night, they hear the chanting and drums of the rituals. The change in deity and new rituals of the tribe made the humans suspicious of the trolls. They fear that the time of peace between the village and the tribe may be reaching its end. They know very well that the trolls are stronger than them, so they try to prepare as best as they can.

If the soul escapes its prison, even without the body, the world is in trouble. The gods weren't strong enough to destroy it last time, and it does not intend to lose again.