Goblins are defiling a sacred temple in the ancient forest. The party has been pressed into service by a dryad to whom they owed a favor. The goblins are mining crystals from the cavern beneath the tree.
Backstory: In the last war of the gods, Mirana, a fertility goddess, was struck by a near-fatal blow and fell from the heavens. She landed so hard, the ground was split and a giant crater was formed. As the war raged on above, Mirana was ignored and left for dead in the crater. Despite being unconscious, her divine powers continued to work to protect her. She formed a protective cocoon of crystal, where she stayed for a millennium until she recovered.
Alternate ending: she died, and the crystals are her coffin or corpse.
Either way, the crystals are highly potent magic. They must not be removed from the forest. Possible plot hook: if they're removed, the forest will die and the magical landscape will change (for the worse for the PCs).