Muspelheim, the second Layer of Ysgard, is similar to the first layer of that plane in that it is made up of shifting rivers of earth that form vast arches through the sky. Unlike the first layer (Ysgard), the earth here is covered in lava and flames as hot as the Elemental+Plane+of+Fire.
Surtr, god of fire giants, dwells on this layer; within it is a portal to the Elemental Plane of Water known as the Silver Eye. The Mystaran Immortal Rathanos has a realm here as well. There is a fortress built by celestials called the Tower of War Triumphant; it guards a portal to the Abyss known as the Black Maw. Somewhere on the layer there is said to be a portal to the Paraelemental+Plane+of+Ice.
Sources: Planes of Chaos, Manual of the Planes, Warriors of Heaven, The Inner Planes
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