Adumbrali

The Adumbrali are a race of two-dimensional, shadow-like entities that dwell in an abyssal dimension. They are only capable of moving in the horizontal plane and cannot move vertically, even in their own dimension, though some have the ability to teleport and reorient themselves to a different plane.

In the RPG...

Adumbrali cannot be harmed by physical weapons, only by magic that affects POW or INT. An Adumbrali attacks by shooting out filaments that engulf their victim, and drain an average of 17 STR and CON. It is not said that these filaments stay attached, and their chance of success is only 30%, so it does not come across as particularly threatening. 

    One interesting tidbit is that the corpse of an Adumbrali victim will be completely dehydrated, and marked with disturbing geometric patterns that have an eerie luminescence and shift position across the skin. Finding someone in this state would certainly provide an interesting plot-hook. 

    Another ability of the Adumbrali is that they can create Seekers, who seem like they would pose more of a threat than the Adumbrali themselves. Unfortunately, neither the abilities to create Seekers, nor teleport, is elaborated upon, so it's unclear whether they are innate abilities or spells. It does say they can "combine efforts" to create a Seeker, which, to me, implies it is a spell. 

    It should be noted that Adumbrali and Seekers did not make it into the 7e Malleus Monstrorum.

In a video game...

With no means of killing an Adumbrali other than magic—and a specific magic at that—I wouldn't personally include them in a game, and even if magic were standard, they are not particularly threatening anyway, so I would still be unlikely to include them.

In a film...

A two-dimensional shadow that can shoot out tendrils could actually be quite scary in a movie, though the protagonists would obviously need a means of defeating them.