The Nemeton is a magical tree stump within the Teen Wolf Universe.
It is located within the Beacon Hills Preserve near where Scott McCall was first bitten by a werewolf in the episode Wolf Moon. The Nemeton is the spot there the convergence of telluric currents intercept.
He says druids picked a large, older tree in a grove to represent the center of the world. Many believed that cutting or harming the "world tree" would bring severe problems like fires, plagues, strife, “death and destruction of all kinds” to surrounding villages. (Visionary)
The tree was apparently cut down sometime after 1943. (The Fox and the Wolf)
The Beacon Hills Nemeton remained dormant, powerless for many years until Derek Hale killed his high school girlfriend Paige among the tree's roots. This unintentional blood sacrifice powered the tree enough to allow Druid Emissary Julia Baccari to survive after being attacked and left for dead by Alpha Werewolf Kali. (The Overlooked)
The stump gained further power when Scott, Stiles Stilinski, and Allison Argent performed a druid ritual to locate their parents. The three students' sacrificial "deaths" fully charged the Nemeton and it is now broadcasting at full power to all the supernatural elements within the Teen Wolf Universe.
In More Bad Than Good, someone pulls out a freshly grown piece of the tree which causes fireflies to form into humanoid beings.
The Nemeton reappears in Season 5 when Jordan Parrish hides the bodies of dead chimeras by the stomp. Dr. Valack also explained that recharging the Nemeton also brought the Dread Doctors to Beacon Hills.
The Triskelion Box
Wood cut from the tree and objects made of that wood still retain some of the Nemeton's power.
Derek, Peter, and Braeden find it hidden in a large chest made of Mountain Ash and filled with sawdust from the same type tree. It is clearly stored in such a way as to keep supernatural creatures out. The top of the cylinder is decorated with a carving of the triskelion (or triskele) symbol, a popular werewolf symbol.
Inside there are five circular recesses, at the time of its discovery each held a single werewolf claw. (More Bad Than Good)
The box seems designed to be used to apply the claws as well. Each claw sits in its own recess and when Peter's hand is jammed inside - it lines up perfectly to embed the claws under his own.
Because of the residual power, the box is used to contain the Nogitsune spirit in its housefly form. (The Divine Move)
Real World Myth
The Nemeton in Teen Wolf is loosely based on the nemeta found in Celtic Religion.
In the real world, a nemeton is any sacred space, often a wooded grove.
The Roman Poet Lucan described a real nemeton located in the south of France in 61 AD -
- "...no bird nested in the nemeton, nor did any animal lurk nearby; the leaves constantly shivered though no breeze stirred. Altars stood in its midst, and the images of the gods. Every tree was stained with sacrificial blood. the very earth groaned, dead yews revived; unconsumed trees were surrounded with flame, and huge serpents twined round the oaks. The people feared to approach the grove, and even the priest would not walk there at midday or midnight lest he should then meet its divine guardian."