Yggdrasil

The tree that Shadow hangs from is Yggdrasil, the World Tree. Yggdrasil is central to Norse mythology. It holds together the 9 worlds and really it's right in the middle of everything. Odin himself hung from Yggdrasil in order to gain understanding and command of the runes.

I know that I hung on a windy tree

nine long nights,

wounded with a spear, dedicated to Odin,

myself to myself,

on that tree of which no man knows

from where its roots run.

Check it out: http://en.wikipedia.org/wiki/Yggdrasil