Yggdrasil

Also known as the World Ash, is a cosmic plane-like tree that spans the astral plane, linking to the prime material plane as well as many of the Outer Planes.

Description

Yggdrasil is a silver-barked ash tree. Its leaves are blue-black and oval-shaped, and its rough bark resembles flaking gray clay. It is made of a type of divine wood that is immune to fire, although fallen branches and leaves can be safely used for cooking fires. Spiraling along the main trunk, planks and sets of steps are carved or mounted onto the tree to facilitate traffic. The tree is surrounded by sky in all directions. It produces seeds, but they are known to be sterile by the elven inhabitants of Arborea and Alfheim.
The entire tree is 23 miles (37 kilometers) tall, with a 15-mile-wide (24-kilometer) canopy. Its trunk is 4,000 feet (1,200 meters) in diameter at the bottom, but tapers down to a diameter of 1,200 feet (370 meters) at the point where it starts to branch. All branches of the tree resemble enormous trees themselves, extending for up to 10 miles (16 kilometers). Many, but not all of them end in two-way portals, which resemble the color pools of the Astral Plane. Since there are many dead ends along the tree's branches, traveling without a guide or directions often results in long detours.
Climbing the tree is difficult, but it is possible to get better footing by wearing spiked boots. The trip is made more perilous by the fact that gravity is not constant along the tree. Throughout most of the trajectory, it points towards the center of the tree's trunk, but it slowly changes to match its destination plane as one approaches a portal. It is usually possible to predict when gravity will change by observing the direction of the moss growth along the trunk.
Day and night follow an erratic cycle around the tree. During the day, sunlight percolates through the leaves, but provides full illumination until, at sunset, the sun appears to move beyond a foggy horizon. At night, the tree is illuminated by countless stars that hang from the branches themselves.
It is possible to reach out for one of those stars by climbing to the end of a branch. If grabbed, the star functions like an Ioun stone, casting a continuous daylight spell around its wearer, and holding on to its magic for up to 30 days after being separated from the tree. However, unless the attempt is performed by or in the company of a dwarf, a worshiper of the Norse pantheon, or a mortal with a chaotic good disposition, reaching out to a star immediately triggers the break of dawn and causes the star to disappear. It is not understood why the stars of Yggdrasil are always out of reach to some people.

Cosmography

Yggdrasil is so massive that it can almost be considered a plane in its own right. It even has certain planar characteristics of its own, such as a mild affinity towards chaos and a mild enhancement of plant- and chaotic-related spells.
The tree's roots are in Niflheim (the second layer of Hades), and its crown is in Ysgard (the homonymous first layer of Ysgard) as described by the Great Wheel cosmology model. Other roots and branches penetrate the Prime planes where Norse deities are recognized, allowing travelers to climb the tree until they reach a portal very similar to a color pool and step through to their destination.
The entry point into Niflheim is usually a favorite among travelers because it avoids the first layer of Oinos, a frequent battleground of the Blood War.
At the other end of Yggdrasil is the first layer of the plane of Ysgard, the home of the rest of the Norse pantheon. Ysgard contains many realms, including Jotunheim, Alfheim, Vanaheim, and the grand realm that gives the layer its name, Asgard. The tree also extends its branches to the second layer, Muspelheim. All of these divine domains are near (in a cosmic sense) to the World Ash for easy access but often require a trek of hundreds or thousands of miles/kilometers to reach the tree.
Once on Yggdrasil, it is said that a trip from the Prime to either Asgard or Niflheim takes at least 100 days, but there is no record of any mortal ever successfully completing the journey. On the other hand, trips between Outer Planes rarely take longer than a week. In both cases, those trips are not without risks, either from the tree's inhabitants, from the risk of falling into the Astral Plane as a result of losing contact with the tree's branches, or from simply walking into the wrong destination by mistake. Falling from Yggdrasil into lower locations is rarely fatal but can cause substantial damage as a falling creature tumbles through branches.
The tree touches all layers of the Beastlands and Elysium, except Belierin, since all portals leading in and out that layer have been sealed. The reasons why Yggdrasil connects with the Beastlands are unclear, since no Norse deities claim realms in that plane. It is speculated that the wilderness and life-giving properties of the plane are sufficient to root the tree there.
Other branches of Yggdrasil connect to the vicinity of Glorium and the realm of the Norns in the Outlands and to Winter's Hall in Pandemonium. The tree also extends a branch into Arvandor in the first layer of Arborea, near a village known as the Gnarl. A branch also extends to Limbo, connecting to a relatively stable forested island known as Pinwheel.
Relatively easy-to-find connections along the branches include Sigil, the Shadowfell, the Feywild, and the Iron Wastes, the 23rd layer of the Abyss. In addition to those, Yggdrasil is connected to every Outer Plane, including Mount Celestia, Mechanus, and even the Styx, but portals to those locations are few and much more difficult to find. The total number of portals is ultimately unknown, since the tree has not been thoroughly explored by any mortal.
The exact exit point of most portals is impossible to predict, since currents from the Astral Plane often shift the position of the branches, causing the tree to bend as if swaying under the wind.
If a traveler dies while on Yggdrasil, their soul is claimed by valkyries. In such cases, resurrection magic only works if granted by a Norse deity.

Inhabitants

The entry point into Niflheim is guarded by Nidhogg, a creature that is sometimes described as a gargantuan, very ancient red dragon, and other times as a serpent. The World Ash and the dragon are perpetually in conflict as Nidhogg tries to sever the inter-planar link to Ysgard by chewing through the roots in order to feed her countless children. Yggdrasil responds by putting down new roots as fast as the dragon can eat them. All this takes place in sight of the feast hall of Hel, Norse goddess and guardian of the dead. The dragon usually ignores travelers coming up and down the roots but fiercely defends herself and her offspring if disturbed.
Viper trees and linnorms are said to be the bastard offspring of Nidhogg. Both infest some of the thinner and lower branches of the tree.
Some of the most important inhabitants of Yggdrasil are the ratatosks, humanoid-looking giant flying squirrels who work as messengers or guides between the planes touched by the tree. In order to hire them for any service, a bribe in the form of pods from Yggdrasil itself is typically acceptable. The ratatosk frequently delivers threats from the dragons in Niflheim to other inhabitants of the tree, which includes giant stags that graze on the leaves and giant eagles that nest on the branches.
The branches of Yggdrasil are also inhabited by numerous tree-dwelling creatures, such as abrians, birds of prey, darkweavers, giant ravens, and spiders. The bark is inhabited by enormous beetles, and the leaves feed entire herds of boar. In addition, bandits and fiends commonly prey on unwary travelers. Some of the most important junctures are patrolled by pers, but they lack sufficient numbers to monitor the entire tree. Angels guard the portals located in some of the upper branches, and yugoloths guard some of the ones in lower branches. A few hags are known to inhabit cottages along the lower branches.
The most frequently encountered travelers along Yggdrasil include bariaurs, Ysgardian dwarves and elves, fire and frost giants, giant owls, valkyries, and winter wolves.