The world of Arcanis holds a less complex cosmology than the default D&D game. To the scholar of the arcane within the world it is known that Arcanis is just one world among many but to the best of anyone's understanding it is the only mortal realm. The others, including the heavens and hells are immortal realms of beings unimaginable.

Common Planes:

The Shadowed Realm

This equates to the shadow-fell or plane of shadow and is a plane known well to the followers of Cadic for he walks this realm and it is from this plane that is followers draw power. The plane its self is thought to be a dark reflection of Arcanis lacking color and light; no sun, moon, or stars adorned the vault of the inky black sky, and all things look as if the color has leeched out, leaving nothing but black and white, which in the dimness are more like "dark black" and "light black".

Inhabitants: The Shadowed realm is called home by a form of undead called shadows, other "shadow" creatures such as the shadow mastiff and shadow dragon, and a race of humanoids known as shades.

The Spirit World

This plane equates to both the Feywild and the astral plane, the plane is a echo of the mortal realm and heavily effected by the events that happen their, sometimes to the outrage of its spiritual inhabitants.

Inhabitants: The Spirit World is the home of the spirits of all things in the material world: living beings, deceased ancestors, spirits of items, and spirits of places, from the biggest mountain to the smallest stone. For example, a lake in the Spirit World may be the abode of a water elemental who is the spiritual manifestation of the lake in the material world.

Settled areas in the material world are, in the spirit world, generally heavily populated by the spirits of the deceased ruled over by the often powerful spirit of the city. These could be the most hazardous, as the ancestors were very protective of their still-living descendants. For instance, a shaman who sought to slay a lady in her palace might first have to get past the angry spirit of her ancestors before he delivered her to them. Seeking the protection of their ancestors, rulers might live in the same palace for generations.

Thus the Spirit World is home to all manner of creature, of all kinds, including dragons, elementals, fey, outsiders, and undead. Spellcasters can summon creatures from the Spirit World to the Material Plane as part of summon spells (like summon monster and summon natures ally).

Ethereal Plane

This is not viewed as a plane unto its self so much as an aspect of the mortal realm, sometimes known as the unseen world, the Ethereal Plane is ever present and home to the invisible and incorporeal. It is said that the Ethereal Plane is the border to the Spirit World which seems likely as ghosts seem to move freely between the Mortal Realm and the Ethereal.

Heavens and Hells

It is well known and accepted that heaven and hell are real and that some spell-casters can make contact with these planes but only the most suicidally fool hardy would dare try to go to these planes fearing gods or demons would kill them instantly for their hubris (they're not exactly wrong).