Bifrost, the "Trembling Road" or Rainbow Bridge, is a Planar Path that connects Asgard to the Prime Material Plane. It is the creation of the Norse Pantheon, and those who don't believe in those gods cannot set foot on it. On the Ysgard side of the bridge, travellers must cross through the city of Himinborg before reaching Asgard proper. Journeys can be made in one to six hours.
It is exactly as it sounds like: a bridge made of light in all the colors of the rainbow, crossing the planes according to the direction of Heimdall. On the Material Plane, it appears to be leading up to a cloud. It is ten yards wide, and mortal magic does not work there. Bifrost has the same effects as sunlight on undead and other creatures of darkness. It also burns and freezes frost and fire giants, respectively.
Sources:
The Northern Reaches Gazetteer, DM Book, page 63
Planes of Chaos, The Book of Chaos, pages 114-115
Manual of the Planes, 1st edition, page 95
Deities & Demigods, 3rd edition, page 165
On Hallowed Ground, pages 140 and 146
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