Heavens and Hell

Heavens
The Heavens is a place created by the Watchers for them to life away from Marn. Achelous created it after he created humankind on Marn. He also brought in existence of the Angels to safeguard the Heavens and to be messengers to mankind.

When the Watchers began to vanish, they put eight select Angels in charge of the Heavens. They named them the Archangels and bestowed upon them great divine powers to keep the Underworld from invading Marn. To this day, the Archangels run the Heavens since the Watchers are no more.

Elwood and Kenlow's home resides in the Heavens (Olympus). They also spend most of their time up there, but Elwood had a period where he was banished, he has recently been allowed to reenter.

Hell
The Hells are more commonly called The Underworld. The Underworld is where the evil souls are sent to repent for their sins (the nonsinful sins were not sent to heaven, it is unclear what happens to them but they do not have to be sent to hell to suffer). It was also created by the Watchers, much like the Heavens and the Spiritworld. After a thousand years in damnation, they are reborn as demons of the underworld and exist to do the bidding of the Dark Watchers. The Underworld was run by Atropos, who was later released from his physical form to become a part of the Underworld. He is no longer here, but his soul remains in the Underworld to run it.

When the Watchers of the Underworld were all killed by William, the Underworld was cut off from Marn and the demons were left to rot. Atropos still exists to torment the sinful souls, and later turn them into Demons, however.