Far Realm
The Far Realm is one of the more obscure planes of Dungeons & Dragons, and basically amounts to the obligatory "Cthulhu Mythos Dimension" for Lovecraftian horrors. It is formally considered an invention of 3rd edition, although its roots lie in obscure D&D lore from the very beginning, in particular the adventure "The Gates of Firestorm Peak", which actually named the plane. #rd edition merely expanded on the details most highly, which, combined with the fact it isn't formally placed on the Great Wheel, is why most consider it a 3e invention.
The Far Realm is generally described as, or believed to be, the origin-point for all aberrations.
Ironically, the Far Realm was about the only major plane to survive completely unchanged when 4th edition switched from the Great Wheel to the World Axis.