The Underneath

The Underneath was the planet of the Shadow Kin, so-named because it was located beneath the rest of the universe. (TV: Brave-ish Heart) It had a blue star and was 9,000 years of space travel away from Earth. The leader of the Shadow Kin ruled from the Shadow Palace. (TV: Co-Owner of a Lonely Heart)

As long as they had an emotional anchor to latch onto, the Shadow Kin were able to create gateways from the Underneath to other planets, such as Earth, using their scimitars. (TV: Brave-ish Heart)

As King, Corakinus led an army in the War of the Underneath, (TV: Co-Owner of a Lonely Heart) beginning with an attempt at invading Coal Hill Academy. (TV: For Tonight We Might Die)