Thanks for providing these. 30 minute creativity exercise, begin.

I guess I could do a dungeon, but that feels like it wastes something that should be more.
– Why do tree roots, which will go absolutely everywhere (ask me how I know), avoid this precise path?
– Whatever this is will have affected the tree, or be intertwined with it.
– It has a fey feel, so we could be dealing with 2+2=orange here. But at the very least, there's a bargain involved.

The tree is the treasure. Let's start there.

The tree is the focus of magic that maintains the natural area around it; not quite mythal class, more subtle but very pervasive in its quiet effects. It draws on the Feywild itself, which is shaped and even powered by stories. Stories that the tree records in the twists and turns of its roots, and readable as lived experiences by those who know how to evoke that magic. It is al this, and more.

It is defended, of course. Part of that defense in obvious. It's a sacred place to the local Grandwood elves, whose aledassen tree-mages commune with their treant partners as they sleep cradled in their boughs.

Part of that defense is less obvious. Drawing so strongly upon the Feywild has created a form of Crossing Point, triggered by walking the path below it in certain ways. There are 3.

1. Walking the path in a certain route creates an illusion of a great treasure at the little island. Some is fey magic, and detectable as illusion. Some is magical and quite real. They fey who live in the roots will do all they can to lure unwelcome visitors along that path, employing attacks, illusions, whatever is necessary to create the hit-and-run chase they desire.

Using overly destructive spells here will cause the roots to come to life and attack you. All of them. Treat the tree as having Spellcraft 23, too, and willing to take offensive action to disrupt overly-destructive spells in advance. The odds of escape after that aren't great for parties below Champion tier.

Touching any of the treasure after walking this particular path shifts non-fey within the maze to the Feywild. The place they end up is not a safe place, as indeed nowhere in the Feywild ever is. It's a one-way trip. Get home from there on your own. If you can.

And the real magic items? Yep. Cursed.

2. Walking the path and touching certain root paths, while intoning the name of the Archfey whose bargain helped create the tree, will summon a projection (sort of like an Aspect) to the little island with the treasure. Her bargain was that in return for the tree, if the elves were ever forced to leave the area their aledassen would convey them to her demense in the Feywild rather than to Lendore/ Evermeet. etc. Until then, true emergencies might make it worth talking to this being. She knows a lot, and inhabits a power class comparable to a mid-tier lower planar lord. But she is a High Lady of the Sidhe, and that is never a casual interaction.

3. Walking the path in another configuration while touching a number of root paths, and intoning a specific ritual that requires more several casters, creates a 30 minute delay before it shifts the tree and everything in its underground passageways into the Feywild. The tree itself has the power to shift back, if properly "piloted" by path and ritual. Yet invoking this power is likely to be interpreted as fulfillment of the bargain in #2. Which is to say, it will happen only at the whim of the Archfey.

4. Walking paths other than these 3 may cause random effects as determined by the GM. Or not. Intruders could wind up baleful polymorphed into animals, have their consciousness and bodies switched, be fed certain experiences, have axes turned into sugar crystal for 30 minutes, hear fey music that entrances them and leaves lasting memories… use your imagination.

USING THE UNDERTREE

Just getting to this tree will be difficult unless you're friendly. The elves will give ground after using illusions et. to make a show of protecting the tree's underground entrance, then depend on the tree's disposal powers to end the threat without deep loss of life. PCs who get greedy are probably in big trouble.

The elves have also been known to create illusions of this tree over other very dangerous underground locations that come with different maps, then lure detected hostiles to those. Rumors of an "everdungeon tree" may have begun circulating. If they do, elven agents will put out rumors about summoning its real treasure. Naturally, this involves following the path of Option #1. It isn't even a lie. The Feywild is an incalculable treasure.

With respect to Army-size threats or overwhelming power, if they elves are given advance warning of such it may trigger their full evacuation with the tree.

For those with better intentions…

PCs who are guided here at the critical point of a major quest may receive very important advice and help from a powerful being (the Archfey, GM choice). One that may take an interest in them, to their future dismay. Or perhaps the Archfey herself is important to something the PCs are doing, and this is how to communicate with her that doesn't require the hazards of a trip into an Archfey's True Demesne.

Mid-tier, which is still not done lightly, might have PCs experience a specific fragment of local history or a specific being's experiences via help from the aledassen tree-mages and communion with the roots.

At a lesser level, they may simply be invited by the elves to pick up a specific cursed item, to make sure it gets to the "right" holder later. It's also a character test, of course – do they take anything else, and trigger option #1?

Good luck, and remember the aledassen saying: as the root grows, so the tree.