This is post 16 out of 25 in the Cygnus Series of a September of Short Adventures. It's also the 4th post in a mini-series about the post-Arthurian Avalon Lost campaign setting.
If the PCs venture southwest of Londinium, into Surrey, Hampshire, or Dorset, they'll begin to hear some mean-spirited comments about those stuck-up Wighters (residents of the Isle of Wight), who are finally getting a well-deserved comeuppance. The dead are now rising on that little island!
Over the past few generations, many Wighters that came over to the mainland exhibited an inflated sense of importance. Many of them paraded their Jute heritage proudly, with seemingly every other traveler being descended from one line of Danish royalty or another. They often demanded more in exchange for Wightish coins and jewelery than their trading partners thought fair. They often brought with them high-quality magic items (vorpal swords being a specialty), and often made it clear that their products are far superior to any that one could find anywhere else.
Now, however, many rumors are arriving across the Solent. There have been dozens of poltergeist-like occurrences in the towns of Medina and Westcowe, and all manner of hauntings in the in the prehistoric mounds and barrows on the west side of the island. Worst of all, the last few mining crews that have been exploring for new veins of precious metals have been attacked by something underground, and the tales told by the survivors hint at the presence of dire things like black puddings, purple worms, umber hulks, and so on.
What's really going on:
No wights, or any other undead, actually. Just a magical experiment gone awry... and trying to take over the world. No big deal, right?
Decades ago, the Isle of Wight was one of several secret headquarters of the dread enchantress Morgan Le Fay. She had built a huge underground complex in the hills outside Medina (on the site of an old Roman fortification that centuries later will become Castle Carisbrooke) to host her most dangerous experiments. The entrance to the dungeon is through a huge tree that sits above the site (cliche noted).
This tree is gnarled and old, with dark, almost blackened bark, but it still is very much alive. In fact, it IS Morgan's main experiment. She initially tried to create a nature oracle to answer her questions and grant her wishes, and it worked for a while. But after she left the island for the last time, many years ago, the tree gradually became intelligent. As it extended its roots farther and farther away, picking up nutrients from scores of miles around it, it has gained both more-than-human intelligence and psionic abilities.
Without any other peers to talk to, it's also grown kind of insane. The strange occurrences around the island are all random psionic effects; either random attacks (against anyone in a target area), or random effects of psionic disciplines reaching out from the extended roots. The tree has access to all Minor Disciplines, but only (so far) a handful of the Major ones. It has used Dimension Door to call forth many of the strange beasties that now occupy Morgan's underground lair. The eldritch roots of the tree were also the cause of the Wighters' luck in mining metals that were especially useful in crafting magic items.
The Tree of Morgan is getting more powerful by the day. It needs only another month or two to extend its roots over the entire island. Another month, and the roots begin to burrow through to mainland England. The tree has become so powerful that it can only be destroyed by the same means that AD&D artifacts and relics can be destroyed. (Recall, however, that one of the ways listed in the DMG is to cause the item to be crushed by Arthur's Dolmen. But is that really a megalithic burial stone, or something else...?)