|Fourth Sinus of Takubar
The Fourth Sinus of Takubar (also known as DOP 9497.15, called Takubar for short) is a pocket realm of Oblivion, and the home of the Cold-Flame Atronachs. It is a realm of extreme cold, where the material bonds of the bedrock slide apart to flow like lava. It is the inversion of Infernace, a realm of extreme heat. Takubar was discovered by the denizens of Coldharbour after Molag Bal banished common Flame Atronachs from his realm. Frost Mares are theorized to originate from this plane.
It can be assumed that Takubar, like Infernace, is a 'collective realm', a highly organized realm of Oblivion created not by a single Daedra, but as a collective extension of its numerous, less-powerful inhabitants. This type of realm is considered relatively exceptional, although they're also rather mundane and uniform.