Hell is a realm within the Tackleverse, located in a dimension distinct from the mortal world.
Hell is mainly populated by a cornucopia of demons, organized in a full-fledged civilization, with at least one large city and a body of government. The ruler of Hell is sitting on the Bone Throne in the Skull Fortress.
Aside from the demons, there are also the souls of the damned, who get transported to Lucifer by the river Styx. In spite of the presence of the spirits of deceased, Hell seems to be distinct from the Land of the Dead, though.
There are three known gateways to and/or from Hell: The dimensional portal built by The Devil and his cronies (destroyed by a portakabin that said portal tried to suck in ), the Great Fiery Elevator , and the eldritch computer built by Eustace Boyce (which strictly speaking doesn't lead directly to Hell, but to a place called the Nexus, from where one may get forwarded to Hell by the Serene Wanderer).
Inhabitants and visitors of Hell:
- Mordawwa, Queen of Hell. Originally being a mortal teenager by the name of Erin Winters, she got sucked in by the Devil's dimensional portal by accident. Fortunately, the demons liked her so much that they didn't immediately eat her, and she won her position on the Bone Throne by defeating Robot Hitler.
- Bob Crowley, who got sucked into Hell by the same accident as Erin, but got eaten by a demon soon thereafter. Being an evil mystic, he was one of the persons who helped to construct the portal in the first place, and seemed to know a bit more about Hell than most mortals - he knew for example that the place is often imagined to be worse than it actually is, thanks to "2000 years of bad word of mouth".
- Manny, a winged member of the Wee Folk, at first a lackey of Crowley, then later of Erin/Mordawwa.
- The former rulers of Hell: Brian, Maximilian + Percy, "Wee" Mike, Toby, Octavian, Silas, Pebbles, Susan and Robot Hitler. Additionally, during Erin's temporary return to her mortal life on Earth, a giant bee named Nigel was declared by Manny to be the new ruler.
- Xaffej the Indefatigable, the guardian of the Great Fiery Elevator. Got defeated by Erin.
- Scientist, Mordawwa's loyal steed, who is actually the spirit of Erin's one true love Eustace Boyce in horse form. While he seems to have retained the intellect of the former brilliant scientist, he can't talk anymore and thus has to communicate via morse code.
- Lucifer, a large humanoid with feathery wings (likely alluding to his often cited origin as a fallen angel), who seems to be distinct from the entity known as the Devil. (Said Devil has been known to have decoys working on his behalf, however.) Lucifer is feeding on the souls from the river Styx.
- Ba'al, Lucifer's metatronic messenger. Didn't get invited to any of Mordawwa's functions because she had always just thought of him as a beam of data.
- The Sheriff, a large black dragon-like entity, who is the Protector of the Underworld. Is a very good friend of Mordawwa, so she got very upset when she found out the that he got murdered. Fortunately, he later got revived by the touch of the hand of Lucifer.
- Archduke Horns, a horned demon who's actual name is VVVVVVVVVVVVVVV. (That's 15 Vs in a row!) Mordawwa renamed him because she didn't consider VVVVVVVVVVVVVVV to be "acceptable nomenclature". Is in charge of a giant weapon called the Arsenal. Also, is actually a smaller demon walking around in a body suit.
- Verkyl, Archduke Horn's cyclopic loyal flunkey. A bit too loyal, perhaps...
- Architekt, a humanoid demon who seems to be the resident voice of reason.
- Amanita, a snarky winged female demon.
- Gorbok, a very strong demon who doesn't talk much.
- Chappy, a cute little yellow demon, who can transform into something a little bit more... viscous.
- Emissary Yama, a Grim Reaper with a rather apocalyptic outlook. Since most Grim Reapers seem to reside in the Land of the Dead , the title might imply that he (or she?) is some kind of ambassador from there.
- Ventus Torbid, who's Stygian hydro-electric dam killed four million, due to Earth-plane physics being wildly unreliable in Hell.
- Several entities defeated by the Sheriff, such as the Obsidian Abomination and the Vengeful Green Planetoid.
- The Serene Wanderer  can be seen helping the Sheriff to repair the Great Membrane after it had been pierced by the Cosmic Shelf.
- The Taxman at one point visited Mordawwa in Hell.
- While there is no direct evidence, it is an obvious assumption that the demon Belial and the Devil are also from Hell. In the latter case, it was part of his evil plan to let Earth get invaded by Hell's inhabitants.
In the (in real life fake) ad pages within Expecting to Fly and Mordawwa: Queen of Hell, several other Hell-associated entities besides Mordawwa herself are title protagonists of advertised comics, such as the Taxman, Architekt (sometimes called "Awesome Architekt"), Amanita and the Serene Wanderer.
Learn more about the religious/mythological concept of Hell here.