Location: The Creeping Forest

This is another theme of adventure that I particularly like. This has the potential to lead a party into a much larger story arc, or it could be left as an inconsequential side quest. Both options are completely possible, and each has their own merits based on whatever's already going on in the story. Credit for the picture to Reddit user Proteon.

                                                               Description:
What used to be a thriving forest stretches before you. The trees are withered and white, their empty branches grasping towards the sky. The ground is bare of life, and the air is dry. Deeper in, you can see remnants of trees, burned black and twisted.

Associated Knowledges:
Nature:

• Most natural life has left this forest.
• Parts of the forest have been burned completely. 
• No nature spirits still live here.

Arcana:

• There is a suspicious lack of magic in the area.
• The burned trees may have been burned magically.
• You can detect very small hints of necromancy around the area.

Geography:

• There are a lot of forests in this area.
• There is a distinct lack of settlements around this forest.
• You have seen some signs of people living here, but they are long gone.

DM Info:
The Creeping Forest was the site of a large forest fire that was started by a group of humans living in the area. They wanted to drive out the local population of centaurs, who were protected by the native treants. Their plan worked, and it destroyed the entire forest. The nature spirits who were harmed by this destruction cursed the entire area, making sure that nothing natural would ever live there again. They also reanimated the treants, but they are now burnt beyond recognition. The humans that lived here either starved to death or were killed by the unnatural guardians.
The forest still remains barren, and most of the spirits have left. The curse remains fully intact, and the treants still wander about, looking for more interlopers. The centaur have also left, and have become nomadic. The treants are different from how they were in life. They are slow because of their burned bodies, but have also become attuned to the fire that killed them. They are inherently evil and bear an undying hatred for any living who would come to their forest. 
Another group of fey don't believe that the forest should be kept barren. It was once a place of power for their people, even though it was damaged by the humans. They are unable to break the curse over the forest themselves, though they have begun a diplomatic conflict with the other spirits that cast the curse. The nature - pun fully intended - of the fey is that they do not take direct action against each other, but rather manipulate events and people to bring down their enemies. They are literally incapable of breaking the curse themselves, though they are fully capable of empowering others to do it for them. This faction is looking for outside help, such as a group of adventurers that could break the curse with the assistance of the fey. 
The goal of the Creeping Forest is to lead into a potential story arc involving a conflict between the groups of fey. It also has the potential to be little more than a one-off adventure, where the PCs go in and kill the evil fey, cleanse the forest, and move on. I personally like to have these types of adventures seeded throughout gameplay. If any one of five or six resonate with the party, then that is the thread of the story that they jump on and take to it's conclusion. The Creeping Forest could also lead to some minor allies and antagonists - if the PCs slight the evil fey by rupturing the curse, they might aid the PCs main enemies in the future. Or the fey that the PCs assisted might owe the party a favor, to be cashed in at the climax of another story arc.