Door Haunts

"Daddy!"
Sarah's father came running into her bedroom "What is it honey?"
"There's a monster in my closet!"

This common childhood refrain is almost true, but the monsters aren't in the closet, they come through the doorways.

Door Haunts are extra-planar creatures inhabiting a dark and twisted realm of five-sided triangles, looming shadows and square lines that parallels the material world. They are drawn to the psychic energies generated by fear and loneliness, which pass from the mundane world to their own twisted dimension through extra-planar cracks that appear whenever a door is left ajar in the dark.

Usually Door Haunts are limited to passively feeding on whatever energies leak through, but certain combinations of angles and shadows allow them to manifest in the material world. They appear as twisting shadows that move from doorway, to corner, to crevice, all dark angles and glittering reflection. Destroying their physical manifestation is as simple as exposing them to bright light, but in the dark they are terrible foes.

Door Haunts have the following characteristics while manifested:

• Extra-planar. Door Haunts are immune to all forms of physical or magical attack, though they can be repelled or sent back to their original plane via an appropriate use of Great extra-planar magic. Exposure to bright light (a good lantern for example), will instantly destroy their mundane manifestation. Their mundane manifestation has no physical component, it is a construct of pure angle and shadow.
• Angle Walk - While manifested, Door Haunts can move instantly between any non-right angle edges within line of sight.
• Psychic Feeding - Door Haunts feed on the psychic energies created by fear and loneliness. A Haunt that feeds for ten rounds can open a Planar Rift.
• Planar Rift - A well fed Haunt can open a rift to their home dimension, transporting themselves and up to two human-sized creatures through.

The dimension of the Door Haunts is a timeless plane of shadow and twisted geometries. Those taken are trapped within constructs of Tesseract and Möbius. Anyone foolish enough to brave this alternate realm must discover a means of opening an extra-planar doorway, construct a weapon of light and angle that will be effective against the creatures of that bizarre plane, then locate those taken and free them from their geometric prison.

"There's no such thing as monsters."  

Those words haunted Sarah's father for the rest of his life. They were the last words he ever spoke to his daughter before she vanished without a trace.