An extra dimension that can be unlocked once the ender dragon is dead.
To progress in the shadow realm you need the elytra, for ender perls don't work.
If the player is to die in the shadow realm they will not drop the elytra because it is irreplaceable.
The shadow realm is an end like dimension that is all dark.
It is filled with copies of mobs but stronger and more powerful (shadow mobs) but these mobs will flee from or attempt to put out light sorcese.
The shadow realm is home to one final boss that will require an elytra to defeat, once it has been defeated it will end the shadow realm dropping an opp item (I don't know what it could be) and the player will spawn in what they think is home but instead a copy of their overworld with mutated and corrupt mobs, almost all life is gone or mutated and the player will have to find their way back too where there portal to the shadow realm would be in their overworld.
Then the player will spawn back home able to travel back to the mutated copy of the over world where the portal to the shadow realm is.