Wilmington was built over the gateway of the Underworld called the Hellmouth. There are at least two in the whole world and it unknown where the other is located. Hellmouths are places of increased supernatural energy.It is the area in which the barriers between dimensions are weak. The Hellmouth has a focal point which serves as a portal between earth and at least one hell dimension that is thought to be Tyker's kingdom. For these reasons, the Hellmouth attracts demons and other supernatural creatures, becoming a "hot spot" for supernatural activity.

The energies emitted by the Hellmouth can have strange effects. Medford, positioned over the focal point, bore most of this supernatural phenomena. The effects of the seal is, in a wise Shaman's words "in this town, over the hellmouth. ..Where the way a thing feels — it kind of starts being that way... for real". For example, a local shy girl who feels invisible, becomes invisible. A boy suffering from stress literally explodes. Here recently the Hellmouth becomes more active than before, competitiveness between social cliques escalates into full-blown war. Inexplicable feats of science become possible. People become drawn to do evil things.

The seal can be describe as a "bidet of evil", expressing incredible evil energy into its surroundings. Standing directly atop the seal of the Hellmouth can cause someone's eyes to turn white as they are compelled to kill, the town have the highest death rates.

Where exactly the Hellmouth leads is a bit unknown, some say it leads deep down into the kingdom of Tyker's domain.