Hellmouth

On 14 October 2016, April MacLean compared a tear in space-time which had appeared within Coal Hill Academy to a Hellmouth. (TV: For Tonight We Might Die)

According to At the Mouth of Madness, one Hellmouth was a wormhole located beyond the Great Astral Fogstorm of Orlock's Spiral. It was believed to lead to Nosfera II, the home of the Vampire Gods. When the Daleks attempted to traverse the Hellmouth, they were reduced to ashes by the Chiroptera. (PROSE: Bats Out of Hell!)

Behind the scenes
Although not mentioned in the DWU, Hellmouths are supernatural locations from Buffy the Vampire Slayer.