From Beneath You It Devours: The Original Hell-Mouth

Detail of a miniature showing the Last Judgement, from the ‘Queen Mary Apocalypse’, England (London or East Anglia), first quarter of the 14th century, Royal MS 19 B XV, f. 40r

Turns out the Hellmouth was well-known from Medieval Illuminated Manuscripts and even church windows detailing the demons in the pit and their victims. Librarian Rupert Giles surely had this manuscript in the Sunnydale Library. Of course the phrase "From Beneath You It Devours" plays a role in Season Seven as the Hellmouth wreaks havoc on Buffy and the Scooby Gang and threatens to destroy the world.


Detail of a bas-de-page scene showing a demon carrying souls to Hell in a wheelbarrow, from the ‘Taymouth Hours’, England (London?), second quarter of the 14th century, Yates Thompson MS 13, f. 139v. 
From the British Library Manuscripts Blog - "Prepare to Meet Your Doom"

The rather horrifying motif of the Hell-mouth was not confined to medieval manuscripts. It appears – most commonly as a hairy, fanged beast – in stained glass windows and wall- and panel-paintings as well. It was therefore visible to ordinary medieval people, for whom owning an illuminated manuscript was an unimaginable luxury, and the prospect of suffering an eternity in Hell an imminent possibility. St John’s visions of the end of the world and the Last Judgement were frequently depicted in ‘Doom paintings’ on the walls and rood-screens of medieval churches. Though mostly destroyed at the Reformation, examples have survived, particularly in East Anglia, some of them ironically protected and preserved by the very whitewash that was meant to obliterate them.

. . . Hell-mouths were commonly depicted in illuminated Apocalypse manuscripts, swallowing up the souls of the damned, while God, the angels and the souls of the saved look on from the safety of Heaven.

See More Hellmouth Illuminations Here

Buffy, Spike and Faith Facing the Hellmouth
For the Final Battle in Season 7