Rex and Hannah pursue some Demons to a mysterious planet at the edge of the Solar System.
Ranquin the Demon
The Guardians of Yuggoth.
This story delves more into the world of H.P. Lovecraft.
When Mr. Lovecraft created Yuggoth in 1930, he indicated it was Pluto, which had then just been discovered. Of course, we know that's not possible now. Therefore, I placed Yuggoth farther out from the Sun, making it currently unknown to human science.