Around the foot of the tree curled the snake Nidhoggr who gnaws at its roots. Nidhoggr is also known in legend as the Dragon who steals corpses or "the corpse tearer". There are few other references to Nidgoggr in Norse myth but its easy to see the similarities with the Serpent in the Tree of Knowledge (of Good and Evil, sometimes also referred to as the Tree of Life) in the Christian Garden of Eden.I hung on that windy tree for nine nights wounded by my own spear.
I hung to that tree, and no one knows where it is rooted.
None gave me food. None gave me drink. Into the abyss I stared
until I spied the runes. I seized them up, and, howling, fell.
The bellowing fire will not scorch them; it will not even touch them, and their
food will be the morning dew. Through the branches they will see a new sun burn
as the world ends and starts again.

I also used the myth of Yggdrasill as inspiration when painting a model to represent the tree itself. I bought a large resin tree at a wargames show a few years ago and here was my chance to use it in a game. The model was excellently detailed and quite easy to paint and I think it made an excellent centre piece during this encounter.
Obviously I didn't have to do all this research or paint the models in order to pull of this particular encounter...but where's the fun in that? There are hundreds of myths and stories in the Norse and Celtic cultures that could be used as inspiration when writing games. I will continue to read and develop these myths and try to find ways to incorporate them in my games.