Yggdrasil

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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 mythos. The tree's roots were in Niflheim (the second layer of Hades), and its crown was in Asgard (the first layer of Gladsheim) as described by the Great Wheel cosmology model. Other roots and branches penetrated the Prime planes where the Norse pantheon was 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 guarded by a gargantuan, very ancient red dragon, named Nidhogg, and her countless children. The World Ash and the dragon were perpetually in conflict as Nidhogg tried to sever the inter-planar link to Gladsheim by chewing through the roots. 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.

At the other end of Yggdrasil was Asgard on the plane of Gladsheim, the home of the rest of the Norse pantheon. Asgard contained many realms including Jotunheim, Alfheim, Vanaheim, and the grand realm that gave the layer its name, Asgard. 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 history does not say that any mortal ever successfully completed the journey. Possible reasons were that the tree was inhabited by giant stags that grazed on the leaves, and the branches held many nests of giant eagles. There was also a giant squirrel named Ratatosk that scurried up and down the tree delivering threats from the dragons in Niflheim.