Nether Void
The Nether Void was a planar void where Leshrac, Kristina of the Woods and Taysir fought during the Summit of the Null Moon. The environment is completely white, without any features.
Description
If planar voids are in fact planes without worlds in them, then the Nether Void was one of the 12 planes inside the Shard of the Twelve Worlds. In the comments in the Ice Age comic, it is described as "some sort of pocket dimension" where it is difficult to cast spells. However, it is equally possible that the Nether Void should be considered the same place as the Abyss.
History
The spirit of Teferi Akosa engaged in conversation with Urza right after the Sylex Blast, in a place that seemd similar to the Nether Void. There was blank nothingness in all directions, populated by hostile dark entities.