The Multiverse is the entirety of all planes of existence, including the Prime Material Plane which encompasses the worlds of Eamon and Earth, the Silvern River, and more distant bodies like Toril and Mystara. Other known planes include parallel material planes that share many characteristics with the Prime; the Inner Planes of elemental energy; the Astral Planes; and the mysterious Outer Planes.
Many names are given to the Multiverse, including the Great Beyond, the Infinite Worlds, the Ever-All, and others.
