Hellmouths are sealed gateways that connect to one of the many hell dimensions. The energies these gateways emanate tend to attract certain types of demons, often leading to large concentrations of dangerous supernatural creatures around them. Hellmouths are created when areas with weak dimensional barriers are breached. These areas can be created via various means, usually as a result of powerful mystical phenomena.
Opening a Hellmouth requires a dark ritual, usually involving a minor blood sacrifice. (This does not necessarily have to be a lethal sacrifice, but a large volume of fresh blood is generally required.) Once opened, the portal will allow creatures and energies to pass freely between the Hellmouth and whichever hell dimension exists on the other side.
Logs featuring Hellmouth: