Shouldn't this be the Underneath, rather than the Underneath?
Talk about it here.
The Underneath was the home of the Shadow Kin. According to the accounts of Andrea Quill and April MacLean, it was a planet. (TV: For Tonight We Might Die, Brave-ish Heart) However, Corakinus claimed that it was not a planet but instead an entity of pure shadow. It was located beneath the rest of the universe, which caused its naming. (TV: Brave-ish Heart) It had a blue star and was 9,000 years of space travel away from Earth. The Shadow King ruled from the Shadow Palace. (TV: Co-Owner of a Lonely Heart)
April MacLean and Ram Singh later visited the Underneath after April vowed to defeat Corakinus and get her heart back. She fought and defeated him, succeeding him as Shadow King. April later used the scimitars to create a portal back to Earth, and also used the portal to command her army to defeat an invasion of flower petals and getting them to close the portal once they had defeated the petals. (TV: Brave-ish Heart)
According to April, the Shadow Kin's culture revolved around the belief that they were cast into the caves of the Underneath by the universe itself. As such, they prioritised the idea of destroying all life to ensure their own survival. (TV: Brave-ish Heart)