The polyderi (/ˌpɒlɪˈdeɹɪ/) is a small, soft-colored stranger that resembles a rounded rubber toy, the exact appearance of which varies between (but not within) populations. Its hollow body is composed of a rubbery material of one fifth of an inch in diameter, and, when squeezed, the polyderi makes a slight squeaking sound. Despite a lack of any mouth or other opening, the tip of the polyderi's snout functions to let air in and out, and anyone holding their palm in front of this area can feel the intake and suction of air as the polyderi is squeezed. Otherwise, its physical properties are identical to any other rubber. It cannot heal from injuries, and when pierced, deflates within a minute.
Populations of polyderi appear on bridges, rooftops, or other high-altitude overlooks. The generation of polyderi exhibits a significant preference for tall buildings, foggy conditions, and contemporary development, but eschews bright lights and loud sounds in favor of more tranquil conditions.
The polyderi generates in the vicinity of previous infestations, leading to the appearance of varicolored populations as individuals pool into a larger arrangement.
The polyderi's disposition is unconsciously spectatorial. It is exclusively found in groups. Only 3-20% of polyderi within a group are capable of movement, and these mobile few push towards their companions as they walk forwards, as though seeking some response which their neighbors are unable to provide. Groups of polyderi do not disperse from the area in which they appear.
It otherwise displays no ability to respond to its environment in any way; with individuals unable to right itself when blown over by a strong breeze, washed into a gutter by a storm, or when grabbed by a stray animal, groups of polyderi eventually become displaced from their companions and litter their environment in a wide radius.
Proximity to the polyderi pulls the eye muscles towards a location within a four mile radius in which a sensitive has died (of any cause). When the sensitive faces away from this location, muscular strain can occur as the pupil points to the far edges of the sensitive's vision, forcing the sensitive to turn to face the spot. This action cannot be overcome with either strength or concentration, but stops once the sensitive leaves the close proximity of the polyderi. This effect can be weakly felt at a distance of up to one-sixth of a mile, leading to eye strain and involuntary twitches.
The polyderi displays no interest in sensitives and does not react to their actions.
At the end of its life, it crumples inward onto itself with a squeak and a final twitch. Its corpse turns hard and breaks apart into small, crusty, pea-shaped fragments over the course of several days.