Congression
Congressions are invulnerable ghosts that will constantly move horizontally in either direction. Upon reaching the end of the room, they will wrap around and reappear from the opposite wall. Some Congressions will reappear in random spots, others will tread the same path constantly.
Many different variations of Congression exist, with slightly different movement patterns. Some move in a sine wave, while others move in a straight line. Some Congression will travel in a packed line, while others will exist on their lonesome. The type of Congression encountered is determined by the room layout.
Isaac's tears will pass through a Congression, but it will block movement and deal contact damage.
Congressions will remain active even once a room is cleared and will also not disappear even once the room is left. The only way to deactivate Congressions is if they spawn in a room with 
Pressure Plates, which will cause them to fade away after the buttons are pressed.
Gallery
Concept art for Congression.
