One of the four major planar paths of the Outer Planes, Yggdrasil — the icon of the Aesir — sprouts from Ysgard and reaches into near half the planes and countless Prime worlds. Yggdrasil, the World Ash, is claimed by the Aesir to have existed since before the very formation of the Planes themselves, rising from the three great wells of Urdarbrunnr, Hvergelmir, and Mímisbrunnr and soaking in their knowledge and divinity; so great is the knowledge drawn into the heartwood of the Ash that Odin's wisdom is said to have originally come from runes which had inscribed themselves upon its bark, offered to the Allfather in honor of his self-sacrifice from its branches.
Yggdrasil is a great silver-barked ash tree with shimmering blue-black, oval leaves and silvery samaras bearing sterile seeds. The bark of Yggdrasil is rough and easily flaked, and pieces of the bark can at rare times be found with runes burnt deep into their inner surface; these runes carry the knowledge of Urdr and Mimir, and can serve as power keys, allowing a priest of the Aesir to call upon their gods even in distant planes. The branches of Yggdrasil extend for miles; near the tree's trunk, they can reach easily dozens of feet wide, but at their most distant end they're narrow enough that a party must walk single-file along their length. The environment around the branches of Yggdrasil is generally mild, if tending more towards cooler temperatures, though the tree does feel a passage of seasons according to the time of year upon Ysgard. During spring or summer, the leaves are so thick as to block most vision beyond a person's own branch, but as autumn approaches, as with any tree the leaves begin to fall clear. When the sky can be seen, it appears as the sky of any standard Prime world — blue in the daylight and starry black at night — though always without any sun or moon. Gentle breezes waft through the air about Yggdrasil, and occasional rain or snow does fall, but one never need worry about the danger of the weather upon the Ash.
The tree is well-populated by animal life, though of course, like the tree, it tends towards the larger. Most commonly encountered amongst the local fauna are massive birds of prey of all varieties (though usually eagles, rocs, hawks, or falcons), herds of boar, burrowing beetles, and hunting spiders. Fungi grows in patches along the branches as well, a common foodstuff for local animals; the most common is a particular variety of black mushroom known for its foul flavor when raw, but popular among certain chefs for the depth of flavor that emerges when properly prepared.
Many intelligent species call the tree home as well, though only two are known to be native to the tree. Most often encountered of these two are the ratatosk, humanoid beings resembling squirrels that reside in warrens dug into the tree, protecting it from anything they deem a threat. Less well-known are the vedrfolnir, a race of wise yet secluded hawk-people dwelling alongside giant eagles in the tree's highest branches. Besides these natives, Yggdrasil also holds significant communities of abrian and darkweavers, having migrated from the Outlands and Pandemonium, respectively; other smaller migrant communities can be found here and there too, including the trading city of Crux near to the root of Urdarbrunnr, a city of émigrés from the now-dead world of Ranais. And of course, members of the Verdant Guild are often found upon the tree's branches, honoring it as the ultimate expression of the significance of the natural.
Each branch holds gravity towards its center, allowing a person to stand on any side; most branches hold flattened surfaces upon their "tops" or "bottoms" for easy travel after so many ages of being traversed by the ratatosk, but some are still as rounded as ever. Still, this gravity only holds for one near enough to a branch. A person knocked far enough from a branch of the Ash will fall towards the roots of the World Ash, eventually — if they cannot catch themselves in time — transitioning into the Astral Plane.
Some branches simply end, but others reach into wide, chromatic pools, indicating a passage into another realm; these pools appear quite similar to Astral color pools, but are considered different phenomena. As one nears such a pool, the gravity of the branch slowly shifts, matching the gravity of whatever lies upon the far end. All such pools allow passage in both directions, and a person can easily take a quick jaunt through to observe the far side. On the far end, the branch almost always continues for hundreds of feet, usually cloaked in fog that prevents the pool from being visible from the ground below; at the ground, such branches tend to take one of two appearances: either a massive root plunging into the soil, or a massive, half-fallen tree.
Outside Ysgard, Yggdrasil is known to reach every realm occupied by or significant to the Aesir; further, it also reaches a single branch into every Prime world which bears worshippers of the Aesir. However, it also reaches a number of places with no connection to the Aesir whatsoever. The following is a list of every known planar destination of the branches of the World Ash, Aesir or otherwise.
- The only known access to the Inner Planes via Yggdrasil, the tree's very topmost branch reaches into the plane of Air.
- The border mountains of Krigala.
- A single connection is known to every layer but Belierin.
- Gray Waste
- A single branch reaches into the stable island of Pinwheel; whether the branch stabilized the island or the stability drew the branch is unknown.
- Though some standard portals to Yggdrasil are known, including beneath a tall statue in the Lady's Ward and within the Hall of Records, it's commonly rumored that a branch of the tree also reaches into the city, said alternately to be either in the rafters of the Carpenter's Guildhall or the city gallows.
- Dead Gods, pp.22-47
- Fires of Dis, pg.7
- The Great Modron March, pg.64
- In the Cage: A Guide to Sigil, pgs.10,51
- The Inner Planes, pg.21
- On Hallowed Ground, pgs.96,138,141,147
- Planes of Chaos: Book of Chaos, pgs.8,42,94,104,106-107,112,115,120-121
- Planes of Chaos: Chaos Adventures, pg.10
- Planes of Chaos: Travelogue, pgs.30,42
- Planes of Conflict: Liber Benevolentiae, pgs.6,17
- Planes of Conflict: Liber Malevolentiae, pgs.5,45
- Planes of Conflict: A Player's Guide to Conflict, pgs.6,9,12
- Planewalker's Handbook, pg.42
- A Player's Primer to the Outlands, pgs.10,30-31
- Uncaged: Faces of Sigil, pg.87