Yggdrasil

The Tree of Life or World Tree exists in a physical location in the Mortal Realm and the corresponding location within the Astral Realm. Within the Mortal Realm, it lies on the larger of the two islands of Asgard.

However, it also touches all the worlds of the Spirit Realm, effectively connecting most of the cosmos. It serves to hold the worlds in place and help regulate the flow of pneuma between them. Moreover, it functions as a hub for the Wheel of Fate.

Idunn tells Odin her people used to call it Djambo Barros, and it presumably has held many other names throughout the ages.

Yggdrasil spans a great chasm where its roots reach into untold depths. A single bridge grants access, one traditionally guarded by Lytir and Ratatoskr (before Odin kills them both in The Shores of Vanaheim).

Each leaf on the tree bears the mark (glyph) of a single soul. When that person dies, the leaf falls. During an epic battle (The Shores of Vanaheim), when many people fall at once, the Aesir witness hundreds of leaves drifting down. Idunn, however, claims one could not otherwise cut down a leaf even with a sword.

Inspiration Notes
Yggdrasil is the World Tree in Norse mythology, connecting the Nine Worlds. Midgard is usually in the center, with Asgard above it. Various monsters and animals live in the tree or under its roots, such as the dragon Nidhogg, who is consuming the roots.

I also obviously draw inspiration from other mythological axis mundi type trees, such as the Tree of Life found in numerous religions.