Yggdrasil

Yggdrasil is the World Ash, a tree of nigh-infinite size in the Astral Plane whose roots and branches join various Outer Planes to the Prime+Material.

Yggdrasil touches the Beastlands, Niflheim, the three layers of Ysgard, Limbo, and Elysium, as well as many other locations. Its roots can be found drinking from the Well of Urd in the Realm of the Norns and from the Well+of+Mimir, and one emerges near the gate-town of Glorium. It also joins various half-worlds that touch nowhere else in the Multiverse. High in the branches roosts the eagle Egder, and at its Niflheim root squirms the dragon Nidhogg and her brood of linnorms.

The World Ash also hosts a major sentient race, the ratatosks.