Yuggoth, also known as Iukkoth, is a planet which features in H.P. Lovecraft's Cthulhu Mythos.
Description
While some sources state Yuggoth to be the planet Pluto itself, conflicting accounts place it as a colossal trans-Neptunian world twice the size of Jupiter; whatever the truth, Yuggoth lies on the very fringes of our solar system.
According to the Revelations of Gla'aki, Yuggoth was the birthplace of a now-vanished species of aliens whose abandoned cities of black stone and emerald pyramids still litter the cold world. Instead of water, the planet is partially covered in oceans and rivers of a warm, black, tar-like substance, it's surface is scoured by savage winds, and the whole dark planet's appearance is enough to eventually drive a man insane.
Since the extinction of the original Yuggothians, a number of entities have made their home there; the Outer God Cxaxukluth and his family settled in an enormous pit there, but all except Cxaxukluth have since moved on. The alien Mi-go have also established a colony on Yuggoth, erecting their city on the edge of the pit inhabited by the Outer God, which means that they must periodically evacuate during those times when Cxaxukluth raises its eldritch head to the surface.