Salvaros, the Planes Pegasus

Connected Planes:
Planes that are connected to Fiore, or the Material.
All these planes are more like "Overlays" to the Material,  and are not truly their own.

Souls.
Souls is the plane in which, well,  the Souls go.  This is a middle state for those souls that did not wish to carry on to the beyond to whatever respective afterlife they may have and instead linger.

Entering the plane of souls allows the interaction with the inhabitants there of,  seeing ghosts,  hearing ghosts,  and dealing with ghosts.   Normally the two planes are held separate but they are inseparable as the two are one in the same but parted by natural laws.
The most separate they can get is as a one way mirror.  Creatures on the plane of Souls can see through to the Material but the Material cannot see through to the Souls.  
There are rare situations in which things can pass between them,   but not a common occurrence.

By entering the plane of Souls,  Salvaros becomes a soul for the time.  Invisible and generally undetectable to those from the material.   But still following all rules for the Souls,  meaning he (or Elen riding him) cannot effect anything on the Material during this time.

Souls are easy to detect for a good plenty mages,  and plenty have found ways to effect them through the veil.   In addition,  certain non-mages (and mages alike) have the natural ability to see ghosts on occasion.  How well they see them remains to be seen as a constant,  but it can vary from perfect clarity to seeing distortions in the air.

Using Souls as a method of stealth can be greatly effective,  but also bears the risk of running into those who can see or at least sense the presence of a Soul.


Shadow.  
Shadow is the plane in which all shadows are bred.  A dark land with only a flaming green sun in the east and a purple sun in the west,  these dim stars provide the means of which the shadows may flourish here without the harshness of true light.

Entering the plane of Souls allows interaction with the inhabitants there of,  seeing creatures of shadow and darkness,  and becoming as a shadow themselves.   There are many a tenacious denizens of the night that are tied to the plane of shadow or have come through to the plane of shadow itself,  while many still walk freely between Shadow and Material even going as far as luring victims into this plane without hope of return.

Being a constant threat to humans and mages alike on the Material,  many groups especially the more religious ones have worked out methods of handling the Shadow.   Charms,  wards against evil,  special substances or simply keeping a room well lit as the Shadow cannot hide when there is no shadow.

Entering the Shadow makes Salvaros a shadow, as well as his rider of course,  which allows movement through the shadows on the material 'a shadow on the wall'.   Granted Salvaros makes for a large shadow,  but shadows in shadows are hard to see.   There are far fewer humans or mages that can detect Shadow creatures but there are many more natural inhibitors to them,  as well as a good number of shadow creatures that can be run into who tend to be far more violent then Souls.

Shadow makes for great stealth,  but has more things to avoid than Soul.  But at least the dangers of Shadow are can be easy to detect while there is no way to tell if someone or something can sense a Soul.  Areas of Light being one of them.


Gloom.
The Gloom is similar to both Shadow and Soul,  it's a place for Souls but also for Shadows and darker still creatures.   In the Gloom there are many hazards and curiosities.  In Gloom,  everything is cast in as you'd expect of the name; a gloom.  Everything is faded and muted,  cast in a rust-hued light.   Where the Shadow sees everything in shadow,  and the Soul sees things as they are,   when entering the Gloom everything changes.   Buildings become ruins,  streets become packed with litter,  the spitting image of a post-apocalyptic habitat.   But,  in the Gloom time is different than in the Material and the natural inhabitants of the Gloom move about in blurred forms as they speed about hundreds of times faster than visitors from the Material.    Certain dangerous entities can perceive through this time difference to prey on those who fall through to the Gloom,  but they are rare.     What you need to worry about are the hazards of decrepit buildings,  a toxic atmosphere that in some locations can be seen as a vapor to even a fog in the air.  In such areas the toxicity is too much and death can be made certain.   And then,  the people themselves.   They do not perceive the visitors,  but their movements can impact those from the material with a life-tearing force,  causing B-ranked damage with each passing,  and there are a lot of passing on the streets.

Entering the Gloom cuts the visitor off from the Material almost completely,  it is one of the loosest connected planes to the Material there are,  but the connection is there.   They have no sense of the material at all unlike Shadow and Soul.   But in turn,  they cannot be sensed from the Material at all either.  In addition,  a daring venture through the Gloom can provide useful in evading persistent pursuers who either can travel planes themselves,  hitch a ride whenever you cast Gate,  or are fast enough or have teleportation abilities with which to keep up whenever you open a gate,  entering through Salvaros's own.