Hellmouths are living, ancient portals that dwell in the fabric of Hell itself, in the mysterious 'between places' separating the planes. They resemble massive worms with a head at each end of their body, which most commonly appears as disgusting rents in the ground, devilish faces, or open orifices. Within each head is a portal that connects with the one in the opposite head. Hellmouths connect the various layers of Hell and are therefore heavily fortified and guarded, with a devil of at least infernal duke status assigned as overseer. A few extremely rare hellmouths connect to other planes, most commonly the Abyss, the sewers of the planar city of Axis, or a world on the Material Plane.
Hellmouths are typically activated by a powerful devil or magic item. Hellmouth travel can be dangerous, but hellmouths' hunger is glacial, so they are largely sedentary. Hell's rulers take advantage of this by using methods to move hellmouths where they want (something that can only be done by divine beings whose realm contains one of the hellmouth's heads) and keep them satisfied at that spot. Hellmouths can defend thmselves by summoning devils, and are skilled liars and manipulators. Killing a hellmouth's head (or banishing it, if it is located outside of Hell) also destroys the portal inside the head, but does not necessarily kill the other; after nine weeks and nine days, the hellmouth reforms in its original location, and each a hellmouth can only be permanently destroyed in a unique fashion.
- F. Wesley Schneider. (2009). Princes of Darkness, p. 5. Paizo Publishing, LLC. ISBN 978-1-60125-189-3
- F. Wesley Schneider and Jerome Virnich. (2015). Hell Unleashed, p. 5. Paizo Inc. ISBN 978-1-60125-757-4
- Robert Brookes et al. (2018). Planar Adventures, p. 195. Paizo Inc. ISBN 978-1-64078-044-6
- Judy Bauer et al. (2014). Lost Treasures, p. 35. Paizo Inc. ISBN 978-1-60125-703-1