Sounds of combat draw the PCs to the ruins of a township by a swiftly-flowing stream. Several trees have grown amidst the tumble-down shells of the buildings, the most majestic of them a willow rising from the remains of the mill.
A handful of goblins surround a dryad, keeping her from reaching her home tree. Three more stand by the great willow, striking flint to steel.
If the PCs understand goblinnish, they hear the biggest of the goblins ask (for the last time) where ‘it’ is, and that if she doesn’t tell them, they’ll burn her tree.
When she sees the PCs, the dryad will plead for help.
A DC 13 Wisdom(Perception) check gives the impression that something is not quite right. If the roll beats the DC by 5 or more, the PC notices that the sunlight streaming through the willow’s leaves is oddly muted, and that shadows beneath its boughs are somehow deeper than they should be.
A black dragonstone has lodged itself in the roots of the willow, washed downstream from Spire Rock, infecting the tree and corrupting the dryad that lives within it. The Greenshae has detected the stone’s presence though she does not know precisely where it is, has dispatched a troop of goblins to retrieve the stone by any means necessary. She will not abide it in the hands of another, as she is highly covetous of her power, and is not yet ready to share.
The corrupted dryad Lasiandra will play the victim for as long as possible, relishing in her newfound ability to speak words that are not true.* She denies knowing what it is the goblins are after, even what it is of which they speak. If she suspects the PCs are on to her (Wisdom (Insight) vs her passive Charisma (Deception) score of 14), if they get too close to the stream, or her tree(and thus, where the stone is hidden among the stones of the stream bank, in the shadow of her home tree’s roots), she will raise up 2d6 twig blights to attack.
* the fey of Thorn's Mystara are normally unable to speak that which is not true. They may twist the meaning of what they say, or direct their words so that they may deliberately be misunderstood by others, but they cannot tell an outright lie. It is unknown if this is a natural phenomena, or if it is a legacy of the ancient pact between the Summer and Winter Queens.
This is (as is everything around here) still very much in draft, and though it is somewhat started for 5e, I purposefully left as many mechanics out as I could so you might use this in your own forest-y encounters, no matter which version of the game you play.