Hellmouth

A Hellmouth an area of increased supernatural energy, according to the mythology of the television show Buffy: the Vampire Slayer and the rest of the fictional universe created by Joss Whedon.

According to the mythology of the "Whedonverse", the Hellmouth is an area where the barriers between dimensions are weak. The Hellmouth has a focal point which serves as a portal between earth and the underworld. For these reasons, the Hellmouth attracts demons and other supernatural creatures, becoming a "hot spot" for supernatural activity.

Spoiler warning: Plot or ending details follow.

There are two confirmed Hellmouth locations in Whedonverse canon: Sunnydale, CA and Cleveland, OH. The plot of Buffy revoles around the Sunnydale Hellmouth, which sits underneath the library of Sunnydale High.

There have been many attempts by groups to open the Sunnydale Hellmouth, such as in the Buffy episodes "The Harvest" and "The Zeppo". In season 7, The First Evil was going to open it to bring forth the Turok-Han, but Buffy and the potential slayers descend through the portal in the series finale, "Chosen" to fight them. Spike uses a magical amulet to destroy the Turok-Han and close the Hellmouth.