Yggdrasil (Eeg-Drah-Sil)

Yggdrasil, called also the World Tree, is a great ash tree so enormous as to form its own plane of existence. Its roots and branches reach into many worlds, and it forms the center of the Norse Cosmology. The tree Yggdrasil has many names on different worlds. It is known as the World Tree, the World Ash, and the Tree of Life
Yggdrasil's many branches and roots lead to other planes of existence. The World Tree is so enormous that nobody has ever mapped all of its paths, though many are known.
Norse Cosmology
In the context of the Norse Cosmology, Yggdrasil is supported by three major roots, each leading to one of the planes of existence: Ysgard the Heroic Domains of Ysgard; Midgard realm of mortals; and Niflheim, realm of the dead. The root at Ysgard is the highest, and Niflheim the lowest.
These three planar connections and hundreds of others are understood the Great Wheel Cosmology, whose travelers climb Yggdrasil to move between the known worlds.
Great Wheel Cosmology
Within the context of the well-known Great Wheel Cosmology, Yggdrasil is understood to have its foundations in the first layer of Ysgard, where the Norse gods live. It has another root in Yggdrasil leading to Niflheim, second layer of the Gray Waste, around which the town of Glorium is built. Another root leads to Pandemonium, where the Norse god Loki has a residence.
The branches of Yggdrasil spread out through the Astral Plane to reach countless other planes. Branches grow into the Outer Planes, where they reach Elysium, the Beastlands, Limbo, and perhaps others. The tips of countless smaller branches lead to hundreds of material worlds where the Norse gods are revered or known.
Transit between planes occurs at color pools, opaque two-directional portals which Yggdrasil grows through leading to another plane.
History
Hanging of Odin
Odin, chief god of the Norse Pantheon, famously hung himself from the World Tree Yggdrasil for nine days and nights, pierced by his own spear. This sacrifice revealed to him the runes of magic. The suffering he endured so impressed the son of the giant Bolthor that he taught Odin nine magical songs, which allowed him to master eighteen previously unknown magical spells.
Prophecy
The wingless dragon Nidhogg is curled around the root of Yggdrasil which grows into Niflheim, and gnaws at it incessantly. According to prophecy, this will ultimately kill the World Tree. Slaying Nidhogg cannot avert this fate. The Norns tend to the tree, but they know the future of all things, and know that fate cannot be changed.
(Image Source: https://www.secretsofthenorse.com/artwork-by-secrets-of-the-norse/)