Oblivion is a name given to the hemisphere of Solus which always faces away from the sun. It is permanently shrouded in darkness and is bitterly cold. Even just a hundred kilometers from Balance, it can easily drop to -50°C, since the only heat it receives is from atmospheric convection. The darkness in Oblivion is extreme; There is no moon in orbit about Solus, so the only natural light comes from the stars. Those species that live in Oblvion, such as the Ice Elves have evolved very keen low-light vision as a consequence.
Oblivion is covered in many vast sheets of essentially permanently frozen ice. Over the millenia, the seas of Heaven gradually evaporated and then precipitated on the far side of Solus. Once this precipitation froze, there was no sunlight to melt it again, so the water of Solus became sequestered in Oblivion. Only by sublimation and small-scale geothermic melting does any of this water reenter the atmosphere.