The black door

The black door is an shadow door that leads to an unknown dimension. It is following Amanda and has either always been her shadow or replaced her shadow somehow.

Continuity
The door appeared for the first time in episode 01 - The Black Door and later returned in episode 03 - The Ghost Library & The Black Door Part II, Voicemails From Darkness - MSG 4: Shadow and Voicemails From Darkness - MSG 5: Dreaming Wrong