Yggdrasil

One magnificent feature of the planes is the great tree Yggdrasil, the World Ash. Rising on the first layer of Ysgard, Yggdrasil's roots and branches extend into many other planes, providing yet another way for travellers to get around. No one has mapped the extent of these pathways, but some of the important paths are known. One of the roots reaches to Niflheim on the Gray Waste, and another stretches to Pandemonium, where Loki (of the Norse pantheon) resides. The branches are no less widespread, even crossing the silvery void of the Astral Plane to reach the very Prime Material. There, it's said, the smallest branch tips touch hundreds of worlds where the Norse gods are revered or remembered. Other branches reach Elysium, the Beastlands, and Limbo, and there are likely to be many more such pathways.

Travelling Yggdrasil's paths requires a traveller to climb through the branches or roots. At each planar crossing, the tree disappears through a portal of shimmering colour, like a colour pool found on the Astral. Only the vaguest of shapes can be seen through the window. To know for certain what lies on the other side, a traveller much step through. The coloured portals are all two-way passages.