Yggdrasil

Yggdrasil, also known as the World Ash, was a cosmic plane-spanning tree that linked the Prime Material Plane through the Astral Plane to the Outer Planes that were important to the Norse pantheon. "Down the tree is easy; back up is hard."

- Tisk the ratatosk

Description
Yggdrasil was a silver-barked ash tree. Its leaves were blue-black and oval shaped and its rough bark resembled flaking gray clay. It was made of a type of divine wood that was immune to fire, although fallen branches and leaves could be safely used for cooking fires. Spiraling along the main trunk, planks and sets of steps were carved or mounted onto the tree to facilitate traffic. The tree was surrounded by sky in all directions. It produced seeds, but they were known to be sterile by the elven inhabitants of Arborea and Alfheim.

The entire tree was tall, with a  canopy. Its trunk was in diameter at the bottom, but tapered down to a diameter of  at the point where it started to branch. All branches of the tree resembled enormous trees themselves, extending for up to. Many, but not all of them ended in two-way portals, which resembled the color pools of the Astral Plane. Since there were many dead ends along the tree's branches, traveling without a guide or directions often resulted in long detours.

Climbing the tree was difficult, but it was possible to get better footing by wearing spiked boots. The trip was made more perilous by the fact that gravity was not constant along the tree. Throughout most of the trajectory, it pointed towards the center of the tree's trunk, but it slowly changed to match its destination plane as one approached a portal. It was usually possible to predict when gravity would change by observing the direction of the moss growth along the trunk.

Day and night followed an erratic cycle around the tree. During the day, sunlight percolated through the leaves, but provided full illumination until, at sunset, the sun appeared to move beyond a foggy horizon. At night, the tree was illuminated by countless stars that hung from the branches themselves.

It was possible to reach out for one of those stars by climbing to the end of a branch. If grabbed, the star functioned 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 was 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 triggered the break of dawn and caused the star to disappear. It was not understood why the stars of Yggdrasil were always out of reach to some people.

Cosmography
Yggdrasil was so massive that it could almost be considered a plane in its own right. It even had 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 were in Niflheim (the second layer of Hades), and its crown was in Ysgard (the homonymous first layer of Ysgard) as described by the Great Wheel cosmology model. Other roots and branches penetrated the Prime planes where Norse deities were recognized, allowing travelers to climb the tree until they reached a portal very similar to a color pool and step through to their destination.

The entry point into Niflheim was usually a favorite among travelers because it avoided the first layer of Oinos, a frequent battleground of the Blood War.

At the other end of Yggdrasil was the first layer of the plane of Ysgard, the home of the rest of the Norse pantheon. Ysgard contained many realms including Jotunheim, Alfheim, Vanaheim, and the grand realm that gave the layer its name, Asgard. The tree also extended its branches to the second layer, Muspelheim. All of these divine domains were near (in a cosmic sense) to the World Ash for easy access but often required a trek of hundreds or thousands of miles/kilometers to reach the tree.

Once on Yggdrasil, it was said that a trip from the Prime to either Asgard or Niflheim took at least 100 days, but there was no record of any mortal ever successfully completing the journey. On the other hand, trips between Outer Planes rarely took longer than a week. In both cases, those trips were 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 was rarely fatal, but could cause substantial damage as a falling creature tumbled through branches.

The tree touched all layers of the Beastlands and Elysium, except Belierin, since all portals leading in and out that layer had been sealed. The reasons why Yggdrasil connected with the Beastlands were unclear, since no Norse deities claimed realms in that plane. It was speculated that the wilderness and life-giving properties of the plane were sufficient to root the tree there.

Other branches of Yggdrasil connected to the vicinity of Glorium and the realm of the Norns in the Outlands and to Winter's Hall in Pandemonium. The tree also extended a branch into Arvandor in the first layer of Arborea, near a village known as the Gnarl. A branch also extended to Limbo, connecting to a relatively stable forested island known as Pinwheel.

Relatively easy to find connections along the branches included Sigil, the Shadowfell, and the Iron Wastes, the 23 layer of the Abyss. In addition to those, Yggdrasil was connected to every Outer Plane, including Mount Celestia, Mechanus, and even the Styx, but portals to those locations were few and much more difficult to find. The total number of portals was ultimately unknown, since the tree had not been thoroughly explored by any mortal.

The exact exit point of most portals was impossible to predict, since currents from the Astral Plane often shifted the position of the branches, causing the tree to bend as if swaying under the wind.

If a traveler died while on Yggdrasil, their soul was claimed by valkyries. In such cases, resurrection magic only worked if granted by a Norse deity.

Inhabitants
The entry point into Niflheim was guarded by Nidhogg, a creature that was sometimes described as a gargantuan, very ancient red dragon, and other times as a serpent. The World Ash and the dragon were perpetually in conflict as Nidhogg tried to sever the inter-planar link to Ysgard by chewing through the roots in order to feed her countless children. Yggdrasil responded by putting down new roots as fast as the dragon could eat them. All this took place in sight of the feast hall of Hel, Norse goddess and guardian of the dead. The dragon usually ignored travelers coming up and down the roots, but fiercely defended herself and her offspring if disturbed.

Viper trees and linnorms were said to be the bastard offspring of Nidhogg. Both infested some of the thinner and lower branches of the tree. Viper trees were also commonly found in Azzagrat, but some specimens were brought to Thay by Red Wizards.

Some of the most important inhabitants of Yggdrasil were the ratatosks, humanoid-looking giant flying squirrels who worked 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 was typically acceptable. The ratatosk frequently delivered threats from the dragons in Niflheim to other inhabitants of the tree, which included giant stags that grazed on the leaves and giant eagles that nested on the branches.

The branches of Yggdrasil were also inhabited by numerous tree-dwelling creatures, such as abrians, birds of prey, darkweavers, giant ravens, and spiders. The bark was inhabited by enormous beetles and the leaves fed entire herds of boar. In addition, bandits and fiends commonly preyed on unwary travelers. Some of the most important junctures were patrolled by pers, but they lacked sufficient numbers to monitor the entire tree. Angels guarded the portals located in some of the upper branches and yugoloths guarded some of the ones in lower branches. A few hags were known to inhabit cottages along the lower branches.

The most frequently encountered travelers along Yggdrasil included bariaurs, Ysgardian dwarves and elves, fire and frost giants, giant owls, valkyries, and winter wolves.

History
A branch of Yggdrasil was a preferred trajectory of the Great Modron March as it moved from Ysgard to Limbo.

Rumors & Legends
The Athar maintained that the Norse pantheon drew its power from Yggdrasil itself. Legends also claimed that the day when Nidhogg finally managed to completely destroy the tree's roots would bring about the end of Yggdrasil and of the Norse pantheon.

The Ratatosk considered Yggdrasil itself to be their patron deity and viewed the tree as a feminine entity.