Plane of Shadow

Shadow is the slow slide into death of something which has been abandoned, it is the order that re-emerges when the tide washes away the sandcastles, it is the pattern of the universe reasserting itself to grind cities to dust.

The Shadow Realm is the dark mirror of Faerie, bleak and desolate where Faerie is lush, ordered where it is chaotic. No grass grows in the Shadow Realm; the ground is covered in light grey dust that coats every surface it touches. Almost everything in this plane is black or colourless, and coloured objects which spend significant amounts of time there begin to fade and turn grey. Before it is complete, this transformation can be reversed by leaving the plane and returning to one without this property.
Shadow is entropy, the slow grinding away of life and colour and vibrancy into dust. It is not overtly hostile to life, just—wearing. It's harder to do things there, harder to cast spells, harder to yell and shout and sing. Not hard enough to stop you doing it, not at first, but subtly, in a way that gradually drains the energy from you until you no longer have the willpower to fight the greyness creeping over you.
Light sources in Shadow are only half as effective as they would be on another plane, including magical light. However, they can be seen for twice as far, due to the contrast with the surrounding gloom. Spells based on light, fire, energy or weather are reduced in effectiveness and have a higher than usual chance of simply failing to work. Spells based on shadow or darkness are boosted as if they were cast with a spell slot one level higher. All other spells have a 5% chance to be reduced in effectiveness. Wild Magic effects do not occur on this plane.
Although there is no objective time dilation between Shadow and Corth, time spent in the Shadow Realm can seem longer and even cause premature ageing.

Geography

Like Faerie, Shadow contains echoes of locations and geography in Corth. The most common manifestation of this is a shadow city, copied perfectly in monochrome grey, standing silent and empty in the middle of a flat grey plain. Other cities or structures appear as ruins.
Shadow is an echo plane, which means that every location in Corth corresponds with a location in Shadow. However, the distance between two locations is not preserved from one plane to the other. Most notably, rather than being spherical, the plane of Shadow is a flat circle, ringed by a border of nothingness. Anything which crosses the borders is never seen again. According to some, it may be gradually shrinking...

Fauna & Flora

Shadow is home to such twisted creatures as cloakers, darkmantles, shadow beasts, and shadow demons.
Creatures which wander unwittingly into Shadow, through portals in dark places, sometimes survive and reproduce, successive generations becoming warped and twisted by the plane’s influence until they become shadow versions of their terrestrial ancestors. These are shadow beasts. The most famous subtypes are shadow dragons and shadow hounds. Able to hide in the shadows to the extent that they can become completely invisible to ordinary sight, shadow hounds stalk the plane of Shadow in packs led by an alpha. They can see into the Ethereal and walk in bright light, but weaken in sunlight.

Type

Dimensional plane

Spell Failure

Light, fire, or weather spells in Shadow must be cast with a spell slot one level higher than normal to achieve the standard effects. Otherwise, their effectiveness will be reduced. Whether this is done or not, every such spell has a chance to fail upon casting. Spellcasters must make an Intelligence (Arcana) check against a DC equal to the adjusted spell level plus 5, with a natural 1 being an automatic failure. For example, a wizard attempting to cast the 3rd-level spell Daylight—adjusted to be 4th level—must achieve a result of at least 9.
Cantrips which fall into this category may be cast as normal so long as one 1st-level spell slot is expended the first time a particular cantrip is cast in a day. Like other spells, fire, light and weather based cantrips require an Arcana check and will fail on a natural 1.