Heck is the dark mystical world, where the forces of evil suffer eternal darnation, when they are destroyed. There are two parts; the lower plains, and the higher plains. Neither part is any less dangerous than the other. Four evil beings rule this world; Him, King Skelehorse, Vorpaxia, and Black-Out.
The lower plains of Heck are inhabited by lower class demons, undead, and creatures of darkness. Though they are strongest and most powerful here, they can be quite formidable anywhere. Thus, demons like the Fluffy Bunch, thrive here. This is where the darned suffer the most torture and torment. The forms of torture shown are often hilarious.
The higher plains of Heck are inhabited by the more mystical and powerful beings of this world. These include dark priests, dragons, mages, spectres, wraiths, and mystic demons. This is where they go to refuel their dark magic. If they visit the lower plains for too long, their powers will fade away.