"All of creation cares not whether you live or die."
Also known as The Second Kingdom and The Dark Kingdom. Hell is, as its name suggests, the home of the Demonic hordes. The forces of Hell on Earth are controlled by the powerful entity known as the Destroyer, who tasked his champion Straga to claim the Third Kingdom, Earth for the might of the demonic legions in their incessant struggle against the denizens of the First Kingdom; Heaven - the Angels.
Other than this, not much is known about the layout of Hell. It is implied it may consist of Nine layers or domains when The Watcher mentions it as such when Samael opened the portal to the Black Throne for War. Demons do occupy realms outside of Hell. Death visited the demon Samael's domain in Shadow's Edge, though little of it was explored beyond Samael's fortress of Black Stone. One of the reasons is that Corruption at that time expanded greatly before Death arrived, even the fortress itself. The other reason is that Death had no interest in coming to Shadow's edge other than to obtain the Demon Key from Samael himself.
Hell is ruled by an entity known as the 'Dark Prince', who was referenced by Vulgrim as a being that Samael may have been a threat to.