Yggrasil is an immense Tree that is central in Norse Cosmology, in connection to which the Nine Worlds exist.
Yggdrasil is placed n the middle of Asgard, where the Gods live. It is an Eternal green Ash Tree; the branches stretch out over all of the Nine Worlds, and extend up and above the Heavens. Yggdrasil is carried by three enormous Roots. The first Root from Yggdrasil is in Asgard, the home of the Gods. By this Root there is a well named Urd’s Well. This is where the Gods held daily meetings. The second Root from Yggdrasil goes down to Jotunheim, the Land of the Giants, by this Root there is a well named Mimir’s Well.
The third Root from Yggdrasil goes down to Niflheim, close to the Well Hvergelmir. It is here where the Dragon Nidhug gnawed on one of Yggdrasils Roots. Nidhug is also known to suck the Blood out of the Dead Bodies. At the very top of Yggdrasil there lives an Eagle. And down by the Roots of the Tree lives a Dragon named Nidhug.
Conflicting scholarly theories have been proposed about the etymology of the name Yggdrasill, the possibility that the tree is of another species than ash, the relation to tree lore and to Eurasian shamanic lore, the possible relation to the trees Mímameiðr and Læraðr, Hoddmímis holt, the Sacred Tree at Uppsala, and the fate of Yggdrasil during the events of Ragnarök.
It is said that at Ragnarök, Odin will ride forth into battle upon his legendary steed, Sleipnir. With his golden helmet and Gungnir in hand, he will face the great wolf, Fenrisúlfr, in battle. The fight will be fearsome and brutal. But in the end, Odin will succumb to the beast as Fenrisúlfr seizes him in it’s jaws and swallows him. That will be the death of Odin.
Ragnarok II artwork by Exileden Ragnarok I can be found here