Realms
The connection between Earth and the other realms is similar in concept to the nine realms in Nordic mythology with Earth in the "middle" and the others branching off from it.
Earth:
Essentially the same as our earth, with the addition of the paranormals. Was originally looked after the god Errastis, but Life took over after he disappeared.
Appiah:
This is the realm belonging to the goddess Love. She's responsible for the soulmate marks. Life also lives here, even though she watches Earth, because otherwise she is overwhelmed by the damage being done on Earth.
Reapers and familiars are forbidden from entering Appiah.
Deletas:
This realm is made up mostly of large cliffs surrounded by ocean. The ocean is populated by sirens and other sea creatures and the cliffs are populated by winged humanoids known as avians.*
The oceans are looked after by the god Cena and the cliffs by Allanis.
Scholars also believe that dragons originated in this realm.
*to my personal knowledge, avians are basically humans with bird wings that sprout from their backs. That being said only one specific family has shown up in the story before so it might be different in some other part of the population
The Underworld:
This one largely speaks for itself. This is where the dead go and it was originally ruled by Death.
The Underworld has it's own ecosystem of flora and fauna, but most of it went extinct in the years following The War. The ecosystem is currently starting to recover thanks to the efforts of Micah and several others.
Living mortals will slowly suffer from asphyxiation without protection. A reapers cloak is one such method
Micah comes to rule The Underworld after Nic and Bellatrix kill Death.
Hapaleon:
This realm is home to the fae and ruled by the god Phierre. The fae live in these massive trees that are big enough for each one to hold an entire city. So far I've only seen one, which is ruled by a council and a fae prince named Ash.
The rest of the realm is suffering from severe desertification, which is populated by a race of dark fae.
Warlocks are unable to enter the fae realm.