Enormous amphisbaenic, semi-living entities that function as portals between different points in Hell—or even connecting to other planes—hellmouths are typically activated by a magical item (such as a melancholic talisman) or at the command of a powerful devil, though certain occult rituals can open a hellmouth as well.
A hellmouth cannot move or be moved save via a divinity whose realm is associated with one of the hellmouth’s destinations. A hellmouth opening onto a plane other than Hell can be forced out of the plane with a successful banishment spell. Though they rarely speak, hellmouths are skilled liars, manipulators, and outright bullies.
If it finds itself in peril, a hellmouth also has the ability to summon a guardian (typically a devil or group of devils, the CR of which can vary according to the importance of the hellmouth’s destination, but should never be less than a challenging encounter of a CR equal to the Average Party Level +1), but it can only do so once per day. The hellmouth’s portal functions as a gate spell used to travel between planes.
Each hellmouth is treated as an intelligent minor artifact, yet unlike typical artifacts, a hellmouth can be destroyed via damage. A typical hellmouth has hardness 20 and 180 hit points. If banished or destroyed, a hellmouth reforms in its original location after the passage of 9 weeks and 9 days. Permanently cleansing a site outside of Hell from a hellmouth’s presence should require a complex and dangerous quest.