Yggdrasil

Borders: Sigil, Mainland, the other realms.

Location: Growing through Ysgard and reaching the realms above and below it.

The Airship Mystic links the Mainland side of the Great Divide to a place near the base of this tree, whose vastness cannot be defined. It is small enough to be recognisable as a tree to mortal eyes, large enough to extend through and link the worlds of which Ysgard is one, and is simultaneously both sizes at once. Portals and gateways from the platforms that encircle the tree lead to the other worlds and one leads to the City of Doors, Sigil, from where worlds Yggdrasil doesn't touch might be reached.

Notable Locations

Alfheim: The realm of the 'light' Elves this is a beautiful forest with a viewing platform from where it can be admired. However travelers rarely, if ever, see any of the inhabitants and there are no shops or quests to draw visitors.
Celestia: The portal to this used to be close to the top of the Infinite Staircase and is now near the top of Yggdrasil. Visitors are closely scrutinised by the guards on the other side of the portal but this and the journey can be worth it for the shops. A tea shop can help fortify travelers for their return journey and the centre of the portion they can visit is dominated by a Temple of Almara.
Hel's Temple and domain: Yggdrasil extends down as well as up from where the airship arrived and in the tunnels formed by its roots a path to the Temple of Hel can be found. Following the road past it leads eventually to a wall with a gate, which it is unwise to pass as beyond it is the entrance to Hel's domain and guarding that is her watchdog Garm. As this great hound is legendarily fated to kill Tyr, and be killed by him, it seems inadvisable for any mortal to attempt to fight him.
'Insel Kalte: Some years ago there were a series of accidents to high places as a berg moved about and grazed them, dislodging snow and ice creatures from it to drop on those below. Eventually the inhabitants of this berg were helped to regain control of its wanderings and then a portal was created so adventurers could brave the cold there and prevent the locals from losing control again.
Jotunheim: The realm of the Giants this is roughly split in three between Ogres, Stone Giants, and Frost Giants. The Ogres live to the east and the Stone Giants within caves whose entrance is to the north. To reach the Frost Giants and their keep requires either passing through the Stone Giant caves or teleporting across a small expanse of water to the narrow bridge that links those domains.
Odin's Reach Tavern: Whether the three lady proprietors are actually the Norns or simply have the same name is unclear, they have displayed impressive knowledge and precognition but running a tavern seems a strange job for Goddesses. A large hole in the floor of the tavern can be descended through to reach a lower portion of Yggdrasil where small Dragons, who might be related to Niddhogr, who legendarily chews at Yggdrasil's roots when bored of chewing on corpses, object to the intrusion.
Underdark: The roots of Yggdrasil are not the only route there and the stretch the portal leads to is a dangerous one, though not as dangerous as trying to pass through a Drow city if your relationship with them is bad. Illithid, Beholders, and other enemies abound in this part of the Underdark and you should be warned that the portal from Yggdrasil's roots only works in one direction. So if you are unable to teleport you would have to find your way to and through the Drow city anyway.