Yuggoth

Location
Yuggoth is a planet located at the very edge of the Solar System. As of this time, it remains unknown to human science.

Surface conditions
Despite its distance from the Sun, it is a habitable planet. Its atmosphere is breathable to humans and the planet is warmed by volcanic thermodynamics (although it is possible that the Old Ones may have played a hand in creating this feature).

The Old Ones colonize Yuggoth and plan to invade Earth
About 5000 years ago, Yuggoth was colonized by a race of beings called the Old Ones. The Old Ones built many cities on Yuggoth, including their main city, called R'lyeh. In a temple in R'lyeh, the Old Ones placed the original Necronomicon, on which all the others were based.

Having already reeked chaos on many world, the Old Ones now turned their attention towards Earth, using Yuggoth as a staging area. Prior to their planned invasion of Earth, the Old Ones dispatched the Mi-go, a subspecies they had created, to Earth to gather intelligence on humanity. However, before the invasion itself could be carried out, the White Lighter Elders and The Source Of All Evil united to fight the Old Ones in a long and bloody war. In the end, the Old Ones were defeated and sealed up in an empty dimension. However, they left behind on Yuggoth beings, that they had created, to act as guardians of the dark planet. These beings began the long vigil of protecting Yuggoth and its secrets from being plundered by outsiders.

With the exception of the guardians, Yuggoth would remain deserted for thousands of years.

Rex and Hannah visit Yuggoth
In the year, 2011, Rex Buckland and Hannah Webster travelled to Yuggoth in order to stop a group of Lower Level Demons who had gone there to use whatever secrets the Old Ones left behind to restore the Underworld.

While there, Rex and Hannah discovered the original Necronomicon, plus various other writings left behind by the Old Ones, in their city of R'lyeh.

Soon after, the group of Demons, that Rex and Hannah came to stop, surprised them. The Demons then tried to force Rex to translate the Necronomicon, so that they could free the Old Ones, in order to offer then an alliance to rebuild the Underworld. Rex refused, knowing the chaos the Old Ones would impose on the universe if released. He tried to tell the Demons that, but they refused to listen.

At that point, the guardian beings the Old Ones left behind to guard Yuggoth attacked, and consumed the Demons. Sensing that Rex and Hannah were not there to plunder Yuggoth, but rather to stop the Demons from doing so, the beings decided to let them go. Despite his curiosity to know more, Rex ultimately decided that Yuggoth and its secrets were best left alone.

Mi-go returned to Yuggoth
In 2014, Rex and Hannah used the Necronomicon to return a colony of Mi-go, that had inhabited the remote hills of Vermont, to Yuggoth.

Rex and Hannah return to Yuggoth
In early 2016, a mysterious illness swept through Arkham and began slowly affecting the town population. Rex and Hannah soon discovered that the illness was tied into the event known as the Blasted Heath. It turned out that the meteor that had landed on the Gardner farm in the late 19th Century had originated on Yuggoth. It had been a piece of volcanic debris that had been hurled into space by an eruption from one of Yuggoth's many volcanoes and had eventually fallen to Earth.

Rex and Hannah then realized that the cure could be found on Yuggoth. Rex and Hannah thus returned to the dark planet and were successfully able to procure the cure from the Mi-go, who were grateful that Rex and Hannah had helped them return home. However, the Mi-go then told the two former Warlocks that the debt was now paid in full, and that no further visits to Yuggoth would be tolerated.

Possible discovery of Yuggoth
These events coincided with the news that scientists had discovered a possible unknown planet at the edge of the Solar System. Whether or not this planet is indeed Yuggoth remains to be seen.