Physical Description and Attributes
The Creeping Shadow appears to not take any defined shape and moves freely as its own substance. This is what makes it particularly strange, as nobody really knows what it is, other than just a shadow. It is assumed to be some sort of non-corporeal entity. It bears some resemblance to the Wraiths, taking on a ghostly, vapor-like material that has manifested itself for some reason, and the leaders of the Facility have captured it; which leads us to assume it has some sort of hostile nature. Being a simple shadow could indicate it is either a spirit or ghost of some kind and affects its victims by possession or fear. Other than a basic description, there is not much else known about this monster.
It is possible that the simplicity and vagueness of the monster's form is simply a result of the production staff's need to fill numerous cubes, and it was not intended to be closely scrutinized. It is also possible that this particular creation was more detailed during production but simply did not show up well at the film's resolution.
It is unknown what item could summon the Creeping Shadow.
The only real time we get to see the Creeping Shadow is in its Cube Prison, where it snakes around its cube, possibly trying to blend into darkness but can't since its cube has a light source inside of it. Although we do not see the Shadow on camera, it could have still been released during the Purge. Since it is a shadow, it simply could have blended with darker areas of the Facility and attacked/killed its victims out of sight.
Shadows have different meanings in horror. Sometimes they represent an evil twin towards a character, or they themselves are their own creatures or a certain form a creature takes. Examples of shadows as living beings include the 1990 film Ghost, Bram Stoker's Dracula, The Princess and the Frog and Nosferatu. Shadows in their own right are a staple in horror even if they aren't living beings. Shadows add a lot to suspense in keeping the dark atmosphere and eerie setting.