Planes of existance

There are infinite planes of existence, or at least too many to count.

Certain Gods exist in their own planes, other share planes for unification of power or for the unification of a pantheon.

All planes can be organized within these three main categories:

Spiritual

Ethereal

Physical

Creatures, objects and everything in between that are natural to a plane inherit its characteristics, but the lower planes require the higher planes to exist, therefore a physical rock inherits ethereal an spiritual characteristics, but its effect in those planes is nil.

Idea came from:

Imagine a big ass skeleton with death looking robes and swords sticking out of it, that is the spirit of "death in battle".

To kill that you need to have a way to inflict damage in the ethereal realm as that is the realm in which the thing is manifesting in, and to survive its attacks you need to be protected in that realm, and to see its full form you need to see in that "frequency", which is the reason that in this universe there are mediums. A lot of magicians can't see spirits and instead learn to use spiritual or ethereal gifts/ skills to affect change in the Physical world (elemental and kinetic mages mostly but some others as well).

With this idea we expand into gods and that side of things, gods can be killed, but also they can't. The concept of the god, or his dominion will continue to exist as those are matrixes of reality, like death, thunder, fertility, etc. But the god's manifestation which holds a personality, can be killed, which will bring great change to the world, which can be catastrophic, but for a lot of it another god will claim that domain or a new manifestation will be formed which presents a new personality, which again, can be catastrophic.

Killing Thor will end that manifestation which the "Thor" that we know will no longer be, but the god of thunder must exist because here, a god is a personalization of a natural concept, as the spiritual realm is conceptual, where the information that constitutes reality resides, through the ethereal realm it gains a more focused form through energy, and the physical realm is where those things gain a definite form, which responds to all changes up the chain, but changes slowly based on the laws of physics, so anything that breaks the laws of physics must be a very energy charged manifestation of the ethereal realm, as the spirit realm manifests through the ethereal to gain a focused and personal manifestation, though spiritual things don't have to appear as an object, animal or person.

For spirits to manifest in the physical realm they must be powerful, as a lowly spirit can be perceived but not directly interacted with, and most spirits can't even perceive the other realms, so the ones that can, are powerful by rule, which usually makes them rule over things or beings.

A spiritual manifestation of the highest degree is an avatar which for the most part is reserved only for gods, as they are incredibly powerful (10/10 power). A stable apparition like a devil, elemental or the animation of a litch's body indicates a very powerful spirit (7/10 at least), a strong but temporary apparition like a ghost which might have limited capabilities in the physical realm indicates a powerful spirit, but because of their physical limitations they pose less of a threat (5/10). And any other spirits under that power level might only be able to relay a message or create illusions as a manifestation, which does take from their life source because ***in the ethereal, your energy is your life source, and without it you cease to be.***

In the ethereal realm, a spirit's energy is its existence, losing energy means it loses existence directly, as in it fades more and more into non-existence (yes this is a big deal). That is why, only very powerful spirits can maintain an apparition or physical manifestation, that takes a lot of energy and the maintenance of it is exhausting.

With a stable source of energy any spirit can elevate their status and influence more of the other planes, that is why ghosts focus on scaring people as they use that fear to draw energy (as energy is simply put the motion of things, or what causes that motion), and with that energy they can grow in power or maintain their status.